Hi,
The problem of the Stereo on one is that there is a need to write to a register to select the channel Right/Left
Instead of just alternate the channel each time a data is sent for example.
The 2nd problem is that the PC need to program a timer at the output frequency, this is extremely slow, here come the FIFO 😀
Plus of course a FIFO (Programmable size) with IRQ When "Almost empty" to have time to start the IRQ at the computer side.
Then, support of ECP/DMA 😀
A Raspberry Pi Pico (Or other microcontroller) can be an excellent to do this, advantage of the Pico is that we can transform this to ZIP Drive or network card emulator as well 😀
But... all those modification will require a specific code to work. Patch of existing software may be not simple.