Hi
I had a look at this. The SoC supports dual audio which can be advantageous at times [1]; but I can understand this can be a nuisance if you don’t want audio coming out of the projector speakers when you have a dedicated system.
You can adjust with alsamixer, but this may not always work depending on how audio is routed. If you can do so, just mute the audio on the projector. It will cause no issues.
[1] No kodi sound output if spotify-connect-web server is active