Hi Glowfish,
I had the same problem, try check the fmod lib for Nintendo Switch. When you a dev build, you need active the lib with “L” at the end. When you remove dev build flag you need use the lib without “L”.
To change the active lib just need check the Switch option on platform settings.
OBS: FMOD is not working on Linux builds, so if your game has a Linux version you will have to wait for them to resolve it yet.