mGBA has reports of sound issues and degraded performance since 0.7.0:
So I tried compiling the RetroArch core for 0.6.3 and surprise: it fixed the sound problems I had with Megaman Battle Network, so I’m quite happy with it.
This is how you can build the RetroArch core:
$ git clone https://github.com/mgba-emu/mgba.git mgba
$ git fetch
$ git checkout 0.6.3
$ mkdir build
$ cd build
$ cmake -DCMAKE_INSTALL_PREFIX:PATH=/usr -DBUILD_LIBRETRO=ON ..
$ make
Then move it to your RetroArch cores directory:
$ cp mgba_libretro.so ~/.config/retroarch/cores/
As far as I can tell, the changes introduced in 0.7.0 were supposed to improve the accuracy on some Pokemon sounds so YMMV by downgrading to 0.6.3 depending on what games are you playing.
Edit: forgot to explain how to change tag