Back in 1994 I was asked to mentor three students who had a "scientific initiation" scholarship and I proposed to add multimedia features to Smalltalk V/286.
While Self has the scheduler in the image built on top of a single "TWAINS" (Transfer and WAit for INterrupt or Signal) primitive, V/286 used the traditional Smalltalk-80 design of cooperative scheduling in the same priority with preemptive scheduling among different priorities. So we just had the highest priority process waiting for a semaphore that was poked by a timer and the actual scheduler was in that process.
I taught the students about different kinds of real time schedulers, like rate monotonic, first deadline first and many others. In the end I implemented all this myself, but it worked as expected. We never did get to the point of playing video or audio in parallel with other things, however.
In the 1994 project I didn't take multiple processors into account, but had a 64 x 68020 Smalltalk machine in a different project which did deal with this. In 2024 it should be part of the discussion.
-- Jecel