What is the method for programmatically adjusting the audio balance (specifically panning between L/R) for Android/iOS?

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For Android, you can use the AudioTrack class to adjust the audio balance by setting the output channels. You can use the following code to pan the audio to the left or right:

AudioTrack audioTrack = new AudioTrack.Builder()
        .setAudioAttributes(new AudioAttributes.Builder()
                .setUsage(AudioAttributes.USAGE_MEDIA)
                .setContentType(AudioAttributes.CONTENT_TYPE_MUSIC)
                .build())
        .setAudioFormat(new AudioFormat.Builder()
                .setEncoding(AudioFormat.ENCODING_PCM_16BIT)
                .setSampleRate(44100)
                .setChannelMask(AudioFormat.CHANNEL_OUT_STEREO)
                .build())
        .setBufferSizeInBytes(minimumBufferSize)
        .build();

float panValue = -1.0f; // -1.0 (left) to 1.0 (right)
// 0.0 will be centered (L/R).
audioTrack.setStereoVolume(1.0f - Math.abs(panValue), 1.0f + panValue);

For iOS, you can use the AVAudioPlayer class to adjust the audio balance by setting the pan value. You can use the following code to pan the audio to the left or right:

// Create an audio player instance
NSURL *soundURL = [[NSBundle mainBundle] URLForResource:@"mySound" withExtension:@"mp3"];
AVAudioPlayer *player = [[AVAudioPlayer alloc] initWithContentsOfURL:soundURL error:nil];

float panValue = -1.0f; // -1.0 (left) to 1.0 (right)
// 0.0 will be centered (L/R).
player.pan = panValue;
[player play];