Same problem here, patched up to perfection now except this.
probably codec related...?
Edit:
Let me be more specific, certain scenes:
mk8.avi (bombing mission)
falllp.avi (midgar plate falling down)
mtnvl.avi (mt. nibel panning shot)
nvlmk.avi (mt. nibel reactor monster)
junair_d.avi & junair_u.avi (lift at junon airstrip)
Probably more, i am up to Junon in my current game and the silence during these action scenes has slightly spoiled what has been an excellent series of mods. i am using ff7music by ficedula and have been using cd tracks from my ost (and switching to psf files for debugging this problem) both work great except some seem to have no sound for these fmv's, i have set the game music to 0 in th settings also.
It seems strange as when i go to my movies folder on the hd and play the offending movies some are silent (which is fine, well you would assume the game audio plays on during these of course of theses) and some do have sound within the avi themselves and both play out during the game silent. To make it even more strange i have sound during the opening avi and several more, it is a really selective problem.