It's still alpha but https://ossia.io/score-web is the whole https://ossia.io (C++ / Qt) compiled to WASM. I haven't implemented file import at all yet though and there are enough UI bugs and latency / performance is too poor to make this nothing more than a fun but useless experiment... but it'll come as soon as I have some time to spend on it :-)

Not sure how much WASM would help, except for very complicated effects and/or sound generators. It would introduce latency as JS<>WASM still needs to communicate since you can't play audio directly from WASM.

The problem with DAWs in browsers is more around latency and precise timing, which browsers don't exactly excel at compared to desktop applications.