Bidirectional Digital Communication over Audio Signal?
It is not clear from your question whether the audio cable is to support an audio signal simultaneously. Assuming it does not, and you already mentioned the data rate is low, you can adapt the hybrid (in telephone terminology) concept.

Check out this link for example: ESP AN-010

I have copied figure 2 here:

I assume the figure is copyrighted by ESP.

You can just feed a low speed (low enough that it can be handled by opamps) signal to "In A" and it should come out at "Out B". Correspondingly, "In B" to "Out A".
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Alternatively, if you have microcontrollers at both ends, it is not that much work to use the built-in UARTs, put in RS485 transceivers and come up with some packetizing and handshaking scheme that works well at low data rate.