Hi, I have found a couple of fixes for the low volume problems.
They worked well for me, so I'm suggesting them here to you.
The first one involves modifying file /etc/mixer_paths_mtp.xml
You have to alter the lines
<ctl name="RX1 Digital Volume" value="84" />
<ctl name="RX2 Digital Volume" value="84" />
<ctl name="RX3 Digital Volume" value="84" />
Bumping up that 84 value to something else. I'm using 95.
I had a couple of issues finding an app able to write that file correctly. Some of them were silently failing, letting me believe the modification happened while it wasn't really the case.
After a reboot, you will find the maximum volume way higher, to the point that at maximum is slightly distorted. The internet suggests that if you use a value that is too high you are risking to damage the speakers. I don't know. Try it at your own risk.
Another possibly simpler thing that worked is an app from the store that works straight away, no reboot required and you can even turn it on and off while an audio file is playing so that it's easier to notice the difference between when it's on and when it's off.
The app is this one
https://play.google.com/store/apps/details?id=com.goodev.volume.booster&hl=en
I'm not a fan of having an additional app always running, but I suggest you to try it because of that instant feedback that you can get.
I've got the information mainly from this video:
https://www.youtube.com/watch?v=FHhvaEtDJvA
The only difference is with the content and the name of the xml file that on this distro is slightly different than the one in the video.
I hope this works for you just like it did for me.