To clarify what I said regarding oss:

oss is one of the two conventionally used kernel driver families for audio. The other is alsa. The N800 uses alsa, but uses esound, a user-space audio mixer, to play multiple streams at once. There is an esound program called esddsp that emulates the oss interface, regardless of the actual kernel layer being used, and mixes that sound with anything else playing...
Of course, Nokia didn't provide this, as you only need it to run things that aren't made to use esd, and they want everything to use esd. (Or maybe they want everything to use Gstreamer, I dunno...) But you can get it from armel sid, and it seems to work.

Package esound-clients is what I used. There's a dependency on esound-common, which I went ahead and installed. It contains only /etc/esound/esd.conf, while the tablet has /etc/esd.conf already. So I installed it, then copied the contents of the original /etc/esd.conf into the new /etc/esound/esd/conf, and now esdplay and esdcat work. (They're what I was after, but I assume that esddsp works too.)

If this works, I'll be so happy.