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Would something like [Alda](https://alda.io/) work for your purposes?  It runs as a server, even when you want to do something like create an audio file, so whether through the REPL or by reverse-engineering it to figure out how to connect to the server, it sounds (no pun intended) like it should get somewhere in the neighborhood.

I don't know the current state of affairs (such as whether it works interactively), but [CSound](https://csound.com/) might also come close.  If you think of Alda as analogous to a MIDI tracker, CSound looks more like a full synthesizer.