Please respond only if you are able to resolve the problem: we need to connect 2 pc for a little data bitrate with audio bandwidth (wire connection). Usb/jack audio connectors. With modem client and modem server (i.e old 56k modem) or whatever...
For example (but you can use other solutions): analog modem using Arduino controller. FSK enconde/decode. 2 modems wired by 2 lines. Input from Pc would be a serial over USB.
Please don't respond if you don't have the capability.