Let me know if this is way to low a priority or if I should put it in Ideas section.
I was looking over the code for [OMX](lhttps://github.com/ricrpi/mupen64plus-audio-omx/blob/master/src/main.c the commit volum section is commented out, saying it doesn't work with Raspberry Pi. I was poking around and it looks like the omx player for raspberry pi which uses OMX as well does support volume control. That code looks to be in C++ instead of C, but has similarities in terms of populating the struct and setting it omxErr = OMX_SetConfig(OMX_Handle, OMX_IndexConfigAudioVolume, &volumeConfig);
That line is from omxplayer
line 812
Any chance OMX was update for RPi and this plugin wasn't aware? My C/C++ skills are out of date especially with linux, so I would not know how to properly get all the source needed to build, make the change. Sadly I have used too much Visual Studio and Windows development. Code would have to be commented out, and also added to call the commented code to increase / decrease.