iPhone: get duration of an audio file

You can use the Audio File Services functions. There's one property to get that should give you the estimated duration. Code:

    NSURL *afUrl = [NSURL fileURLWithPath:soundPath];
    AudioFileID fileID;
    OSStatus result = AudioFileOpenURL((CFURLRef)afUrl, kAudioFileReadPermission, 0, &fileID);
    Float64 outDataSize = 0;
    UInt32 thePropSize = sizeof(Float64);
    result = AudioFileGetProperty(fileID, kAudioFilePropertyEstimatedDuration, &thePropSize, &outDataSize);
    AudioFileClose(fileID);

You can check the docs here

Correct code is

NSURL *afUrl = [NSURL fileURLWithPath:soundPath];
AudioFileID fileID;
OSStatus result = AudioFileOpenURL((CFURLRef)afUrl, kAudioFileReadPermission, 0, &fileID);
Float64 outDataSize = 0;
UInt32 thePropSize = sizeof(Float64);
result = AudioFileGetProperty(fileID, kAudioFilePropertyEstimatedDuration, &thePropSize, &outDataSize);
AudioFileClose(fileID);

outDataSize should be Float64 not UInt64.