Using an audio cable (or similar) to create unidirectional communication from a secure server

Using audio to communicate between 2 servers sounds way more complicated than necessary, not to mention error-prone and fragile. You'd end up implementing some kind of modem in software. The complexity of that "modem" could be anything from morse code on up, but I don't think it's going to be terribly efficient no matter what.

There are so many other methods of unidirectional communication you could choose from instead:

• You could use a unidirectional serial connection: only TX and ground are wired, leaving RX unconnected.
• You could use a half-duplex ethernet connection. Same idea, only this time you have to wire the RX pair to a dummy dead-end ethernet hub in order to spoof link integrity.
• You could have an additional secure device on a network between the two systems that forwards traffic in only one direction (basically a firewall).