$ dd if=/dev/dsp | ssh username@host dd of=/dev/dsp
The default sound device on Linux is /dev/dsp. It can be both written to and read from. If it’s read from then the audio subsystem will read the data from the microphone. If it’s written to, it will send audio to your speaker.
This one-liner reads audio from your microphone via the dd if=/dev/dsp command (if stands for input file) and pipes it as standard input to ssh. Ssh, in turn, opens a connection to a computer at host and runs the dd of=/dev/dsp (of stands for output file) on it. Dd of=/dev/dsp receives the standard input that ssh received from dd if=/dev/dsp. The result is that your microphone gets output on host computer’s speaker.
Want to scare your colleague? Dump /dev/urandom to his speaker by dd if=/dev/urandom.
It works the other way as well:
ssh -C user@ipaddress arecord -f dat -D plughw:1,0|aplay -f dat