Well, a little more time spent on Google and such, and it works now. :)
Thanks for your responses, everyone.
Here's what I did:
1. Set up ~/.asoundrc according to
<http://www.alsa-project.org/alsa-doc/doc-php/asoundrc.php?module=emu10k1
>. Changed "card" from 0 to 1.
2. (Since I was using Rhythmbox to play MP3 streams...) Used Synaptic
to remove gstreamer's OSS package, and installed the gstreamer ALSA
package.
3. Used GConf to change "audiosrc" and "audiosink" in
/system/gstreamer/0.8/default/audiosrc from "osssrc" and "osssink" to
"alsasrc" and "alsasink".
4. Clicked the play button in Rhythmbox.
5. Done. :) No reboot or anything, didn't even have to restart the
program. (Compared with Windows, where you'd have to reboot at least
once, probably...)