One idea would be sending the servos control via MIDI, which would give you much more control over.
You just use a free MIDI + Audio software, which sends the original Stereo Music to the speakers and the servos control via a MIDI cable. Them you can use the Serial-to-MIDI converter found on the link below to use the USB connector as a MIDI device.
http://www.spikenzielabs.com/SpikenzieLabs/Serial_MIDI.html
MIDI is very easy to handle, I can send you some codes if you want, or just google for Arduino MIDI Input.
Wk