SigmaTel C Major Audio configuration help needed on Xubuntu.

Freda's computer is her mother's old Dell XPS M-170 gaming laptop. (This is identical to the Inspiron 9300 but with glowing blue bits on the lid.) It's still a pretty damn sweet piece of kit and we frequently remind her that she is the luckiest little girl ever to grow up in a house of computing machinery.

Given her fondness for advertising-filled Flash game sites of dubious provenance, I'd rather it wasn't running Windows. So I put Xubuntu on it. This works perfectly, including sound (SigmaTel C Major Audio) ... but controlling the sound is proving problematic.

There are three speakers: two tweeters in stereo at the front and a woofer in mono. In Windows, these all work together. (You can't change the balance per-speaker, but at least they all go.) In Xubuntu, the sound control panel lets me put sound out either the treble speakers or the bass one, not both at once.

Does anyone know of a sound control panel for GNOME or XFCE that actually lets me switch on and balance all speakers at once, as I wish?