Crazy, never knew this was a thing:

spkmodem:

This module provides support for outputting GRUB console information over an audio output. This output can be fed into another computer’s sound input and decoded using the spkmodem_recv utility. Note that this will slow down GRUB’s performance.

https://www.gnu.org/software/grub/manual/grub/html_node/spkmodem_005fmodule.html