BTTF sometimes dialogue sounds don't work
 

Hello, I have a small problem in back to the future.

This is rare but in some conversation scenes the dialogue from certain characters isn't spoken. I was wondering if there is a work around as subtitles don't work for all the scenes.

I'll attach a dxdiag as well if it helps.

EDIT: apparently the file size for the dxdiag 21kb or so is too big for these forums :p