No, the video plugin would not affect speech. Video plugins affect only the graphics, not the sound. SPU plugins are what you use for sound, music, and, hopefully, speech. What SPU are you using?
The ones that I, personally, reccomend are Iori's, Andy's, or Pete's.
Give one of those a try and see if it helps.
I've had no speech problems so far with Iori's, Andy's and Pete's and I recommend Andy's the most for this game as it seemed to sound the best, while Pete's was dropping a few sounds. Are you sure you have all the sound options enabled? Most notably, XA sound.