Volume Level Oddity

My XviD movies with have low volume. The only ones I've used so far have AC3 audio, and I guess this is an AC3 audio filter issue since the same thing occurs in other players. However, I've noticed that setting the playback speed to less than 100% (99% will suffice) results in a higher sound level. Can anyone shed some light on why this would be?

(BTW, my sound hardware is a TB Santa Cruz outputting analog stereo to a Bose speaker system).