AVAudioPlayer sound files playing out of sync when mixed - iPhone

I am trying to play 6 seperate .mp3 sound files using instances of AVAudioPlayer. The sounds are playing at the same time but they seem to be playing out of sync or at slightly different speeds. Does anyone know why this may be?

Here is how I initialize a sound:

 NSURL *musicurl = [[NSURL alloc] initFileURLWithPath:[[NSBundle mainBundle] pathForResource: @"SoundLoop" ofType: @"mp3"]];
music = [[AVAudioPlayer alloc] initWithContentsOfURL: musicurl error: nil];
[musicurl release];
music.numberOfLoops = -1; // Loop indefinately
music.currentTime = 0; // start at beginning
music.volume = 1.0;
music.meteringEnabled = YES;

and here is my AudioSession code:

AVAudioSession *audioSession = [AVAudioSession sharedInstance];
audioSession.delegate = self;
[[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayback error:nil];     
[[AVAudioSession sharedInstance] setPreferredHardwareSampleRate:44100 error:nil];
[[AVAudioSession sharedInstance] setPreferredIOBufferDuration:30 error:nil];

UInt32 sessionCategory = kAudioSessionCategory_MediaPlayback;
Float32 hardvol = 1.0;
AudioSessionSetProperty(kAudioSessionProperty_AudioCategory, sizeof(sessionCategory), &sessionCategory);
AudioSessionSetProperty(kAudioSessionProperty_CurrentHardwareOutputVolume, sizeof(hardvol), &hardvol);
UInt32 doSetProperty = 1;
AudioSessionSetProperty(kAudioSessionProperty_OverrideCategoryMixWithOthers, sizeof(doSetProperty), &doSetProperty);
[[AVAudioSession sharedInstance] setActive:YES error: nil]; 
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Could it possibly have anything to do with the bit rates of the sounds? or that I am using .mp3?

Thanks.

I found the solution to this problem by using the playAtTime: method of the AVAudioPlayer class:

   NSTimeInterval shortStartDelay = 0.5;            // seconds
    NSTimeInterval now = player.deviceCurrentTime;

    [player playAtTime:now + shortStartDelay];  //these players are instances of AVAudioPlayer
    [player2 playAtTime:now + shortStartDelay];
    [player3 playAtTime:now + shortStartDelay];

Using this will allow all the sounds to play asynchronously and in sync.

You should check out the MultiMedia Programming Guide. The "Using Audio" section has tons of helpful information. http://developer.apple.com/library/ios/#documentation/AudioVideo/Conceptual/MultimediaPG/Introduction/Introduction.html

This sounds like it relates to your issue:

When using hardware-assisted decoding, the device can play only a single instance of one of the supported formats at a time. For example, if you are playing a stereo MP3 sound using the hardware codec, a second simultaneous MP3 sound will use software decoding. Similarly, you cannot simultaneously play an AAC and an ALAC sound using hardware. If the iPod application is playing an AAC or MP3 sound in the background, it has claimed the hardware codec; your application then plays AAC, ALAC, and MP3 audio using software decoding.

To play multiple sounds with best performance, or to efficiently play sounds while the iPod is playing in the background, use linear PCM (uncompressed) or IMA4 (compressed) audio.

Here's another bit that claims what you are doing should be possible but it seems like Apple is throwing in a caveat with "the most processor-efficient multiple playback" line. I would think that if the processor is strained that would lend itself to not keeping things perfectly in time.

Starting in iOS 3.0, nearly all supported audio formats can be used for simultaneous playback—namely, all those that can be played using software decoding, as described in Table 1-1. For the most processor-efficient multiple playback, use linear PCM (uncompressed) or IMA4 (compressed) audio.

In terms of debugging, you should start with two audio sounds and see if there is an issue. Then work your way up to the 6 and figure out if there is a distinct point at which the problem starts to occur. I would also find (or make) some audio tracks in the formats that Apple recommends (PCM or IMA4) and do the same testing with those. Doing these two things should help you narrow down what the actual problem is.