BASIC and PCM aren't a good match for each other.
PCM takes up gobs of space... would consume large swaths of ROM real-estate, and would also be crappy (of necessity).
PCM requires lots of shepherding by the CPU, which BASIC is not really suited to doing, except in cases where
the entire sample fits into the FIFO and can thus be just dumped in and ignored
The BASIC command to play a PCM clip larger than 4k takes hold of the program until the entire clip plays.
Also, remember that these extensions to BASIC aren't really going to be super useful as background music player functionality. BASIC really doesn't do that. You really need to have some kind of IRQ-driven code that is loaded somewhere in memory and sent commands from BASIC. I've written a very simplified interface to the Zsound music player that does this. It's mostly a proof-of-concept / demo of how to do such a thing with Zsound. It doesn't do anything that's required of such a thing to be truly viable, such as modify MEMTOP - so doing a lot in BASIC with the existing module would actually overwrite the code with BASIC variables at some point.
Thus, having a "drum kit" in ROM is really not going to be the useful thing that it would seem to be.
BASIC is ultimately a sort of "cradle" where programming is more easily picked up from scratch, but then you quickly outgrow it, unless you just wanna show off by doing things in BASIC that others thought impossible, or else the task at hand is really not depending much on performance or timing, so who cares what it's written in? (record album organizer, anyone?)
The objective here is to make playing little ditties, and audible prompts into something much more immediately accessible for novices, not to be a real sound API.