sounds in movieclip timelines should play as long the initial keyframe that instantiates the sound, plays (or the sound is set to stream).
What type of symbol is it, a button, movie clip or graphic?
What is the sync setting for the sound file?
I have tried it as a graphic and a movieclip type of symbol. I added a new layer, dropped my imported sound onto the stage, I can see the waveform in the layer, hit enter and the sound plays. Go back to the main scene, hit enter... no sound. When dropping a sound like that there's no way (I think) to set the synch type. So I went back into my symbol, removed the layer with the sound on it, added a new layer and in the first frame, set frame properties to play the sound. Hit enter and it plays, exit out of the symbol and try playing the scene... no sound. Went back in and tried the different synch settings, made no difference.
The only way I could actually get the sound to play from the scene is by adding it to the scene's main timeline itself. Fortunately synching up isn't too bad (so far) because I add the sound to the frame properties of the keyframe where I play the symbol from. But my concern is eventually adding more sounds, some overlapping, some not... plus having to always remember to add that frame property if I use that symbol again, etc...
I've struck the same problem. Except that when i play the symbol with the sound on the main timeline they are no longer in synch!
Did you find a solution to this?