I don't have any in-game sound either. However, on the very first title screen and on the login screen I do get sound.
linux, 1.3 GHz AMD Duron, 512 RAM, GeForce2 MX 200 (using nvidia's driver, not the open source one), SBLive 5.1 (which works fine in all other application; alsa driver)
I know that the minimal specs say that around GeForce3 is the minimum, but would this really affect the sound?
Edit: It seems I do have sound sometimes, but sooner or later it disappears. Once gone it never returns unless I restart planeshift.