I’ve got a 5 second piece of a song in a TBS file where I have a few photos of heads with their mouths opening and closing to the words.
When I play it back to check it it sometimes plays right and about half the time plays back much faster. The music plays back correctly, just the animation is wrong.
By playback do you mean the play button inside the software. If that is the case that is somewhat normal, be aware that the in software playback is actually loading images on the fly so you will most likely have an offset when you preview things there as the sound will not stop to wait for the images to be loaded.
If you want to have a very accurate representation of what your scene sync should be you should try to do a Preview Scene or Preview Movie. the Quick Preview will be somewhat accurate but since it is an swf playback it will first need to buffer the images and the sound before playing things in sync. It may also help to check the Streamed checkbox in your sound to fort synchronizing of element to sound.