SDL has alsa support, it just might not be being used. You can force SDL to use ALSA by setting the SDL_AUDIODRIVER env variable to "alsa".
For example, start tremulous like this:
SDL_AUDIODRIVER=alsa tremulous
The default audio driver is dependent on your SDL library. It seems like ALSA should be the default, but on my distro (Slackware) the default is "dsp" (a.k.a. OSS)