The author probably hasn't tried them (otherwise they would be in the Readme), but there are actually a couple of existing Forth-like audio languages. Quite the niche. I'm one of the most avid users of one such language called Sporth, for which I made an online live playground at https://audiomasher.org/

The Sporth author created multiple stack-based audio languages and I haven't even kept up with all of it. He has some interesting projects at https://git.sr.ht/~pbatch/

In any case, sapf looks very carefully designed, and the addition of functional elements inspired by APL seems like it complements the stack approach very well. And the examples actually sound good to my ears, which isn't a requirement but generally a good sign. I'm tempted to get cracking on a WASM build right away...