This article explains how to use macros, self-modifying code, cycle-counting and data on the stack to play four channel music on the one-bit speaker of a 1981 PC. (Interrupts must be disabled to avoid jitter: so I think this pushes all the buttons for items of interest.)
http://www.reenigne.org/blog/8088-pc-sp ... -its-done/
"there's 3 layers of code here: the actual 8088 mixer code, the v-instruction code in the stack and the h-instruction code interspersed into the song data"
Oh, and watch out for patching instructions which are already prefetched - the old version will execute.
"Debugging is still tricky, though - breakpoints don't work so well when your entire program is executed from the same 15 byte memory region!"
The player cuts in at about the 6m35s mark of the demo video - prior to that it's all beeps:
https://www.youtube.com/watch?v=yHXx3orN35Y&t=6m35s