PC Junior / Tandy 1000 (1983)
Since their PC gained a certain success, IBM took itself courage and tried to compete with established home computers like the C64. They recognized the limitations of the PC in matters of video and sound, however what they created, was not really outstanding. The PCJr didn't know expansions in form of plug-in cards, and instead one had to put expansion adapters at the front - a new expansion was connected to the last one. That could turn into a true rat tail, if you wanted to extend something more!
IBM PCJr - (c) www.old-computers.com
But the main problem of this machine was the keyboard which ran via infrared, and instead of the promised several meters only sitting quite near its front was possible, and so so it was practically free of any sense.
One thing it shined in was its sound: using a chip by Texas Instruments, it was possible for the machine to pass 3 sound channels and one noise channel with 16 volume steps each! Looking at these specs today, this doesn't sound much better than the capabilities of the first Gameboy, but for the PC-world, it was a real enrichment!
Meanwhile, also Tandy developed a clone, which they wanted to praise as "PCJr compatible". However IBM withdrew its model at this time, because although their second version solved most deficiencies of the first one, only few were sold - possibly because of the bad expandability.
So Tandy only titled its model "MS-DOS compatible". In contrast to the PCJr however, this model could spread quite far, and so the music playback is supported by quite a few games on these systems.
A later expansion in the Tandy TL/SL made the direct playback of digitized samples possible with simultaneous utilization of the synthesizer!
Recreating the sound in hardware
This may come as a surprise, but the british seller Lo-Tech actually sells empty ISA boards that can be equipped to work as Tandy sound adapter in a regular DOS ISA machine! The only limitation is software - if programs didn't ask the user for the sound option, they may simply check for "Tandy" in the BIOS - hence some programs may only work when patched.
Have a look at the product page: Lo-tech Tandy Compatible Sound Adapter.