I have the exact same problem.
Even while waiting for a match to start and people are doing global voices, i see my screen lag while the sound plays.
I've been trying unsuccessfully to get the game to run smoothly all of the time. It runs smooth sometimes, and a very few particular maps I just can't get to run smooth worth a damn (Cavern, Tropics).
Its really annoying.
I know I should be getting better FPS, I've tried all the tweaks in the "Big Monkeys Tweaks" thread and all the sites dedicated to squeezing out performance. Nothing seems to help much more than a couple FPS, and the sound makes me lag always.
Specs:
AMD Barton 2500XP
ASUS A7N8XD
512MB DDR
BFG Geforce 6800 OC 128MB
Note: I've even tried overclocking my cpu and gpu, nothing helps.