Using audio instrument cable for 1MHz transmission
Rather than messing around with RF modulators and demodulators, I would recommend simply encoding the MIDI data differently, so that its bandwidth does not impinge on the audible range of the analog signal.

For example, you could Manchester encode the data at 2&times; the MIDI baud rate &mdash; i.e., sending each async bit twice. This would give you a signal that's centered at 62.5 kHz, with most of the information in a band that's within &plusmn;15.625 kHz of that.

On the transmit side, send the signal through a 45 kHz - 80 kHz bandpass filter before mixing it with the audio; this will eliminate "splatter" into the audio passband.

On the receive side, use the same filter to block the audio before sending the signal to a threshold circuit ("data slicer") and then to your Manchester decoder. The output of the decoder will then feed an ordinary MIDI input with no additional work. A simple low-pass filter @ 25 kHz or so will eliminate the MIDI signal from the audio path.