It already had Audiophile card selected as input. The only thing the audiophile mixer allows me to do is adjust the output gain, which makes all the instruments (and my keyboard) get louder or softer together.

Let me see if I can write down how I think things work and tell me if this is correct. The midi output from my keyboard goes into the midi input on the audiophile. BAIB has as it's input MIDI driver the audiophile card. So BIAB takes the midi from my keyboard and uses the Coyote synth to produce audio output. That audio output is mixed with the audio output from the backing track and sent through the audiophile output to my stereo. So it seems like the only part of the process that has separate control over the audio volumes is BAIB.