ok, for some reason I was sure I'd typed that in already, but this time it worked.
Turns out the sound isn't affecting the framerate. It still slows in the same spots. I think it's the particles causing the slowdowns. The first obvious bit is at the start when quakemanthingy goes around the first corner and lobs a grenade at 3 zombies. at the point of explosion it all slows. The screenflashing also seems to freeze it briefly.
I'll need to have a look through that config thread.
hmm.. now for some reason after using nosound, the timedemo doesn't work properly.. it just continues on thru all the demos and doesnt provide a report. beh, I should play with something else.