Problems with sound file on main timeline

I have a Flash movie that basically consists of two things on the main timeline - a movieclip inside which are nested several animations, and a sound clip dropped into the first frame. The sound clip is a voiceover which lasts for around a minute.

When I play the movie locally, the voiceover matches perfectly with the animations (which is the reason why I put it on the timeline, rather than streming it dynamically). However, when I play it online, the voiceover and animations go out of synch.

Why is this? Is the sound clip slowing down slightly? I thought that sound on the timeline would play at the same rate regardless of straming issues.

Does anyone know of problems that can occur with delays or changes in speed of sound on the timeline that might only manifest themselves online?

I hope that’s a clear description of my problem. I can elaborate if need be.

Thanks in advance to anyone who can help.