We maintain AMY, which is a great synth SDK for many platforms. It lets you build synthesizers like analog Juno-6 style, or 80s FM like the DX7, or samplers, or whatever you want. It comes with lots of default presets and examples to try, so you don’t need to really understand synths to start making cool sounds in your games.
We recently made a Godot addon for anyone to make synthesized music in their games programatically. It’s all open source and free to use. We’d love feedback on it! amy/docs/godot.md at main · shorepine/amy · GitHub
A special feature is that it works seamlessly on web exports as well! The engine has a webassembly port so it switches to that on web exports.
Please try this out and let us know how it works for you! We have a discord for more guidance: shore pine sound systems