Thanks for the custom soundtrack link, InSutterWeTrust. I'll give it a shot and see what songs that package has. I don't have a huge problem with the EA TRAX (or whatever EXTREME TO THE MAX name EA came up with for the game's soundtrack) but it gets a little repetitive having "WE ARE THE WARRIORS OF TIME!" beaten into my brain every few whistles.
I use TVersity to stream music and vids to my 360, thanks to this
helpful post from Igottago. It's not that hard to set up - just read the FAQ for it - and so far, it seems to have less issues with detecting files on one of my network drives, loads files faster, and with less buffering.
As for the sound being so low, yeah...not much you can do about that, aside from using GoldWave or something to jack up the volume of your audio files, and turn up the arena music slider/turn down some of the others, as someone already mentioned. Even then, it can still be hard to hear, but that can be explained away (in my head, at least) by the fact that this is supposed to be a TV broadcast, of sorts...the music isn't going to be 100% clear.