Really weird sound problem

[the8bitwizard]

I have a 12" powermac G4, and I have a weird sound problem. I have an external set of speakers and a USB headset that I use varying on the situation. I switch between them using the sound prefs panel and use the volume controls on my keyboard. Recently though, when both the headset and external speakers are plugged (as usual), the volume controls on the keyboard only affect the headset. All other volume controls work normall (menu bar, sound prefs panel). when I switch the output to USB headphones, everything works normally. its only when I switch to the ext. speakers (they're plugged in via the headphone jack)