Audio: Change Volume of samples in byte array

Question

I'm reading a wav-file to a byte array using this method (shown below). Now that I have it stored inside my byte array, I want to change the sounds volume.

private byte[] getAudioFileData(final String filePath) {
    byte[] data = null;
    try {
    final ByteArrayOutputStream baout = new ByteArrayOutputStream();
    final File file = new File(filePath);
    final AudioInputStream audioInputStream = AudioSystem.getAudioInputStream(file);

    byte[] buffer = new byte[4096];
    int c;
    while ((c = audioInputStream.read(buffer, 0, buffer.length)) != -1) {
        baout.write(buffer, 0, c);
    }
    audioInputStream.close();
    baout.close();
    data = baout.toByteArray();
    } catch (Exception e) {
    e.printStackTrace();
    }
    return data;
}

Edit: Per request some info on the audio format:

PCM_SIGNED 44100.0 Hz, 16 bit, mono, 2 bytes/frame, little-endian

From physics-class I remembered that you can change the amplitude of a sine-wave by multiplying the sine-value with a number between 0 and 1.

Edit: Updated code for 16-bit samples:

private byte[] adjustVolume(byte[] audioSamples, double volume) {
    byte[] array = new byte[audioSamples.length];
    for (int i = 0; i < array.length; i+=2) {
        // convert byte pair to int
        int audioSample = (int) ((audioSamples[i+1] & 0xff) << 8) | (audioSamples[i] & 0xff);

        audioSample = (int) (audioSample * volume);

        // convert back
        array[i] = (byte) audioSample;
        array[i+1] = (byte) (audioSample >> 8);

    }
    return array;
}

The sound is heavily distorted if I multiply audioSample with volume. If I don't and compare both arrays with Arrays.compare(array, audioSample) I can conclude that the byte-array is being converted correctly to int and the other way around.

Can anybody help me out? What am I getting wrong here? Thank you! :)

Answer 1

Are you sure you're reading 8-bit mono audio? Otherwise one byte does not equal one sample, and you cannot just scale each byte. E.g. if it is 16-bit data you have to parse every pair of bytes as a 16-bit integer, scale that, and then write it back as two bytes.