Before I lose my train of thought, one other possibility if you re-visit these modules. These low-cost RF modules appear to come in either 315 or 433 MHz versions...I think that's a SAW resonator on the TX side and I can read the frequency on the can. It could be that one receiver is less susceptible to the MP3 harmonics/noise than the other.
I'm focusing on the what to do with the spare audio track right now, but my to-do list now has to hook up the 4-channel RF receiver to see if it plays well with the $1 MP3 player driving the 82 cent class-D audio amp. My 4-channel TX is 315 MHz.