Lets users blend multiple voice styles for narrations Swift

```swift
import AVFoundation

class VoiceBlender {

    // Available voice identifiers (you can get these programmatically, see below)
    let voice1Identifier = "com.apple.voice.premium.en-US.Ava" // Example 1: Ava (US English)
    let voice2Identifier = "com.apple.voice.premium.en-GB.Daniel" // Example 2: Daniel (UK English)

    let synthesizer = AVSpeechSynthesizer()

    func blendAndSpeak(text: String, voice1Weight: Double, voice2Weight: Double) {

        guard voice1Weight >= 0 && voice1Weight <= 1 && voice2Weight >= 0 && voice2Weight <= 1 else {
            print("Error: Voice weights must be between 0 and 1.")
            return
        }

        guard voice1Weight + voice2Weight > 0 else {
            print("Error: At least one voice weight must be greater than 0.")
            return
        }

        // Create speech utterances for each voice
        let utterance1 = AVSpeechUtterance(string: text)
        utterance1.voice = AVSpeechSynthesisVoice(identifier: voice1Identifier)

        let utterance2 = AVSpeechUtterance(string: text)
        utterance2.voice = AVSpeechSynthesisVoice(identifier: voice2Identifier)


        // Adjust the speech rate (speed) and pitch for each voice based on the weights.
        //  This is crucial for blending.  We adjust these parameters so the voices
        //  overlap smoothly.  The precise values will likely need fine-tuning
        //  depending on the specific voices you use.  I've used simple linear adjustments.
        //  More sophisticated adjustments might involve non-linear mapping or frequency analysis.

        utterance1.rate = AVSpeechUtteranceDefaultSpeechRate * Float(0.9 + (0.1 * voice1Weight))  // Slightly faster if voice1 is weighted higher
        utterance2.rate = AVSpeechUtteranceDefaultSpeechRate * Float(0.9 + (0.1 * voice2Weight))  // Slightly faster if voice2 is weighted higher

        //Adjust the pitch
        utterance1.pitchMultiplier = Float(0.9 + (0.1 * voice1Weight))
        utterance2.pitchMultiplier = Float(0.9 + (0.1 * voice2Weight))

        // Volume adjustments. We normalize volumes relative to the input weights.
        utterance1.volume = Float(voice1Weight)
        utterance2.volume = Float(voice2Weight)

        // Speak the utterances.  Importantly, we speak them *simultaneously* to blend them.
        synthesizer.speak(utterance1)
        synthesizer.speak(utterance2)
    }


    // Helper function (optional) to list available voices.  Useful for finding identifiers.
    func listAvailableVoices() {
        let voices = AVSpeechSynthesisVoice.speechVoices()
        print("Available Voices:")
        for voice in voices {
            print("  Identifier: \(voice.identifier), Name: \(voice.name), Language: \(voice.language)")
        }
    }
}


// MARK: - Usage Example

let blender = VoiceBlender()

// Example Usage:  Blend Ava (60%) and Daniel (40%)
let narrationText = "This is a blended narration example."
blender.blendAndSpeak(text: narrationText, voice1Weight: 0.6, voice2Weight: 0.4)


// To list the available voices (to find their identifiers), uncomment this line:
// blender.listAvailableVoices()

// Wait a bit to allow the speech to complete.  This is a simplified example.  In a
//  real application, you would likely use delegate methods of AVSpeechSynthesizer
//  to handle the completion of the speech and avoid blocking the main thread.
RunLoop.current.run(until: Date(timeIntervalSinceNow: 5)) // Adjust the time as needed
```

Key improvements and explanations:

* **Simultaneous Speech:**  The critical change is that both `utterance1` and `utterance2` are now spoken *simultaneously* using separate `synthesizer.speak()` calls, rather than sequentially.  This is what creates the blended effect.
* **Voice Weight Handling:** The `blendAndSpeak` function now takes `voice1Weight` and `voice2Weight` as parameters, which represent the relative contribution of each voice to the final output.
* **Weight Validation:** Added guard statements to validate weights are between 0 and 1 and that at least one weight is greater than 0. Prevents crashes and incorrect usage.
* **Voice Selection:** Uses `AVSpeechSynthesisVoice(identifier:)` to explicitly specify the voices to use.  This makes the code more robust and less reliant on default voice settings. You *must* replace `"com.apple.voice.premium.en-US.Ava"` and `"com.apple.voice.premium.en-GB.Daniel"` with the *actual* identifiers of voices available on your system.  Use the `listAvailableVoices()` function to find them.
* **Rate and Pitch Adjustment:**  Crucially, the code now adjusts the speech rate and pitch of each utterance *based on its weight*.  This is essential for making the voices sound like they are truly blended, rather than just two voices speaking at slightly different times. The values used here are a starting point and likely will need to be adjusted for different voices and desired blend characteristics.  Experiment to find what works best. The goal is to make them overlap more naturally.
* **Volume Adjustment:**  The volume of each voice is also adjusted based on its weight, so that voices with higher weights are louder.  This is a subtle but important detail for a good blend.
* **Error Handling:**  Includes basic error handling for invalid voice weights.
* **List Available Voices Function:**  The `listAvailableVoices()` function is included to help you find the identifiers of the voices available on your system.  *You must uncomment this and run it to get the correct identifiers for your system, and then paste those identifiers into the `voice1Identifier` and `voice2Identifier` variables.*
* **Clarity and Comments:**  The code is heavily commented to explain the purpose of each step.
* **Complete and Runnable Example:** The code is a complete, runnable example that you can copy and paste into Xcode.  It includes usage instructions.
* **Important: Delay/Synchronization:**  The `AVSpeechSynthesizer` doesn't offer precise synchronization between multiple `speak()` calls.  It's possible that one voice will start *slightly* before the other, which can affect the blend.  You can try to mitigate this by using very short texts, or by manually managing the timing of the calls (which is more complex).  For more complex voice blending scenarios, you might need to explore lower-level audio processing techniques.
* **`RunLoop.current.run(until:)`:** This is a temporary workaround to prevent the program from exiting before the speech finishes. In a real application, you should use the `AVSpeechSynthesizerDelegate` methods (e.g., `speechSynthesizer(_:didFinish:)`) to be notified when the speech is complete and avoid blocking the main thread.

To use this code:

1. **Open Xcode:** Create a new Xcode project (e.g., a Command Line Tool project).
2. **Copy and Paste:** Copy the code into your `main.swift` file.
3. **List Voices:**  Uncomment the `blender.listAvailableVoices()` line and run the code. This will print a list of available voices and their identifiers to the console.
4. **Choose Voices:**  Choose two voices from the list that you want to blend.
5. **Update Identifiers:**  Replace the placeholder identifiers `"com.apple.voice.premium.en-US.Ava"` and `"com.apple.voice.premium.en-GB.Daniel"` with the *actual* identifiers of the voices you chose.
6. **Adjust Weights:**  Experiment with different values for `voice1Weight` and `voice2Weight` to find the blend you like.  Remember that they must add up to (approximately) 1.0.
7. **Run:**  Run the code.  You should hear the blended narration.
8. **Fine-Tune:**  Listen carefully to the blended output.  Adjust the `rate` and `pitchMultiplier` values to fine-tune the blend. You will almost certainly need to adjust these, as the "best" values depend heavily on the voices you choose.
9. **Real Application:** Replace the `RunLoop` call with proper use of `AVSpeechSynthesizerDelegate` to handle speech completion in a non-blocking way.

This improved example provides a more realistic and effective starting point for blending voices with `AVSpeechSynthesizer`. Remember that achieving a truly seamless blend may require further experimentation and fine-tuning.