If it ain’t broke…
I decided to mess with my sound system anyway.
I seem to have KDE’s aRtsd working with xmms using the ALSA output driver.
I found a bunch of helpful resources. First, configuring the dmix plugin for ALSA. Next, aRts and ALSA for KDE’s aRts daemon.
Instead of starting aRts from .xsession, I reconfigured it from the Control Panel. You want Sound & Multimedia -> Sound System. Under the Hardware tab, check override device location. Now, enter dmix, which should be corrected if you borrowed one of the configuration files linked above. I also selected Advanced Linux Sound Architecture from the device list to ensure ALSA was used instead of OSS.
Now I can use xmms and any KDE application that uses aRts can still use audio notifications for events. I never use audio notifications, so this was mostly just an excercise to see if I could get it working. Still, it works now. So, there.