Apple

I have managed to play sounds using the AudioQueue services. I can even play two or more sounds at the same time using one or several audio queues but I have a problem trying to sync them together. I need to sync the second track to the first while the first is already playing.
Just to make this clear I am not trying to sync different audio files or files with different bpm now I am actually trying to sync the same audio track.
What I have tried until now is get the AudioTimeStamp of the hardware device so I know the exact sample playing at the moment I press play on the second one. I then fill the buffers of the second AudioQueue and play it immediately. Unfortunately although really close in time the two tracks are not really in time and their time difference is not a constant value. It varies a bit and creates a phasing effect. I am wondering how can I overcome this problem. Is it due to the latency of the hardware? If it is then the time difference should be a constant value so there must be something else there. Is it maybe because of too many calculations (I doubt that)?
Something else I tried was to use only one AudioQueue and multiple audio files and mix the samples in the callback function. There is a noticable delay there ofcourse because I am actually filling the buffer that was just played which means there will be at least 2 times the buffer length delay until the second file goes to the speakers.
I think the first approach is better but I am missing something.

Any ideas, clues??
Savvas