fast forwarding a song using AVAudioPlayer

Use the currentTime and duration properties.

i.e. to skip forward

NSTimeInterval time = avPlayer.currentTime;
time += 5.0; // forward 5 secs
if (time > avPLayer.duration)
{
    // stop, track skip or whatever you want
}
else
    avPLayer.currentTime = time;

Similar for going backwards, only compare currentTime against 0, instead of duration