http://paste2.org/p/196546
That's the log. What should I do? The version from getdeb also segfaults, so don't suggest that. Using ubuntu 9.04.

I guess its a crash by the sdl variant of nexuiz.. seems one of the sound related changes introduced a small thread problem into 2.5.1. That log file will sadly not really help. There are debugging versions of the engine available that you can run in a debugger or set it to create a coredump. That might help more.. But people are looking into it already.

OK. So what should I do to be able to play nexuiz?

The next logical step is to use the glx version instead.
nexuiz-linux-glx.sh