The nvsound module is OSS type, i.e., a kind of driver different of ALSA sound driver, so to make xmms work with nvsound select the oss output and it will sound just great!!!
The problem is that arts, and all kde sound, will not work. It actually sounds but in a very bad way. You can try it by selecting in the control center, in the audio section, the Open Sound System Multitrhead (or something like that), and then make a sound test (It is awfull, I know).
So, now you can use xmms with OSS, and mplayer too at the same time.
For the guy who asks about the drivers, the alsa driver use the sound card by SOFTWARE, i.e., it is not using all de HARDWARE capability of the chip, what is the reason why most of us bought it
. So yes, you need the Nvidia driver.