To be more specific...
I spent months and months trying to figure out how to do this in a better way than my first "solution" (which was to put each song's backing track as a separate "player" containing each individual audio file, and then using the mixer to adjust each song's volume--it works, but it's overly complicated and a clunky solution).
I wanted more simplicity, so this was my simpler solution: use patches to save several presets with different level adjusts below and above 0 db. To do this, I used the "channel strip" level adjust and ignored the rest of the functions (EQ, compression, etc).Then for each song that needs volume increase or decrease, apply that specific patch preset to the specific song.
There is probably an even easier way, but this is much better than my original method. I've read a whole bunch of people asking this exact question on the internet and most answers were that it can't be done. Well, it can, just not as directly as if Presonus would actually add this functionality to the audio clips themselves (as you can do when multitrack recording), which they could do and it would be much easier.