Hi,
First, in case you were unaware, any wav/mp3 volume you set in a UPG gets saved with it if you have kept the 8x updated, I believe, so UPGs are not a problem.
Having said that, in the System Menu, 14.4, there is a place to set the wav/mp3 volume that should affect the playback volume regardless of what set you may be using since it is a system setting. Whether that setting overrides a volume in a saved UPG I don't know but would doubt it.
So, to try it,
a) Press and release the Menu/Write button
b) Use the big data knob to scroll through the menus to get to the System menu (menu 14)
c) then, press the knob in to get to the sub menus, then rotate the knob to get to menu section 14.4
d) Press the knob twice to get to the wav/mp3 volume setting.
e) rotate the knob to the volume you want and test without turning the 8x off. You might have to experiment. Don't be surprised if all wav/mp3 volumes from different songs vary even by doing this because a lot depends on the volume they were recorded at. Doing these steps simply sets the base volume settings for wav/mp3 files. That's why UPGs are a good way to go.
f) Hold in the Menu/Write button until the save menu comes up. It should say System, if not rotate the knob until it does
g) Press and release the Menu/Write button twice to do the save
Don't worry about screwing the wav/mp3 volume up, if you don't like it, just go back and put the wav/mp3 volume back to what it was.
Always make sure you have the latest OS update. You can see it briefly when you turn on the 8x as a small number at the bottom of the initial screen. The latest one is 2.01.077. In any event, try the steps above, I think they will work. If any thing I have mentioned is not available, it would be because you don't have the latest update. Tough to keep track of what updates added what
hope it works