You can do a neat trick with this if you set up IP over AX.25, particularly with softmodems. Since you've got IP you can do SSH or TLS over it, right? At least, if you set all the timeouts really long, because some of those packets take a while at 120 bytes per second.

So then you can tune the tones to be a little off the normal frequencies of one side, and play them through speakers with two PCs connected together. When you ssh from one to the other, you will hear the establishment packets and the flurry of packets for every keypress pingponging backwards and forwards between the two systems.

Absolutely brilliant for demonstrating how things like TCP works with retries (plug a mike into it too, shout some interference) and how UDP doesn't, and stuff.