No more volume control

I have a Dimension 8250 running XP SP3.

I recently disconnected a set of USB Soundsticks with which my system worked perfectly.

I'm now running a line from the headphones out port to a small stereo.

This sounds great but, I no longer have control of the volume.

Even going into the audio control panel, none of the volume controls work.

This wouldn't be a big deal because I can use my stereo's volume but I really

miss being able to use the mute control on my keyboard.

Any ideas?