You haven't mentioned what audio quality you need, or how high transfer speeds the FSK is intended for or how long cable lengths and what cable qualities you requires.

Use two FSK frequencies well above the audio frequencies.

Then use a low-pass filter to extract the audio and a high-pass filter to extract the FSK.

The higher up in frequency you place the FSK, the harder it will be for your signal line to transfer the signals long distances, but the easier it will be for your low-pass/high-pass filters to extract the two signals.

Or consider digitizing your audio signal and then send this signal and the input to the FSK encoder as a single combined digital signal, possibly as a higher-speed FSK signal.