None of the solutions provided in this thread worked for me, audio in SIP always got choppy after only a few seconds into the call and somehow every new PR made things worse. I think it might have something to do with all my routers running DD-WRT, but anyhow ...

Today I installed the bleeding-edge wl1251 wifi driver by lxp. This one really does wonders! I made two - albeit short - SIP calls and one longer Skype call and all three of them went quite smoothly. No choppiness at all. There was some lag on the SIP calls but that's most likely caused by low upstream (64 kbit/s).

I'd really urge everyone who still has troubles with SIP to give this one a shot.