Hello,
I haven't tried VC injection, but it seems to be more complicated than just using an emulator and a game file.
Anyway, I've just understood why the sound is not working on some mgba installations and not on others!
mgba is relying on some DSP code and you need to dump a dspfirm.cdc from your system.
There are some tools on github which can get this file:
I can't post the direct link here because I'm a new user, search for DspDump by Cruel and DSP1by zoogie.
The first one didn't work for me, the second did (I'm using luma)
Once it's installed, you'll get sounds in mgba!