Open a terminal and copy-paste this into it:
sudo aptitude install libsdl1.2debian-pulseaudio
And hit enter.
You will be asked to confirm with your password. This might effect a lot of packages and some will need to be removed, answer 'yes' then asked.
Depending on how you installed OpenArena you will need to disable OpenAL in Setup in the game, it will freeze one last time then closing.
You cannot remove pulseaudio from an Ubuntu 9.10 system without breaking everything.