Story

I've taken a standard PC, freeware Audacity, and manually generated both Morse Code and Binary data in a simple .wav file using 20kHz - 22kHz "sound" with some fade in/fade out to clean up 'tics'. When played you cannot hear it (the dog goes nuts though). I then used my iPhone and a sound spectrum analyzer (free app) and monitored the inaudible frequencies.

BINGO...

A partition type virus combined with modem type software (but modified to use inaudible sound) could easily perform communication between PCs.