If I take a full instrumental song that's say, 2:30 and replace one of the quick :10 second arena songs, will it fade out after 10 seconds like the default song, or will it play all the way through?
Don't think anyone knows because there has never been a way to edit the ingame dittys. Not that I know of anyway (have never seen any ingame sound patches in any of the lives). Either way, whether it's a full song (that the game engine fades in/out) you can just replace the song (say 2:30) with 2:30 of the song you want to put in, or if it's not the full songs just small clips of them replace them with a 20 second clip of it that has fade ins and fade outs done by yourself.
I'd like it to fade out, and if it won't, does anyone know of an mp3 editor that would allow me to shop the full mp3 into just a 10 second clip to play during the games?
Yep that's easy. There are lots of programs that can do that type of thing. Steinberg Wavelab is what I use for wav editting but you can get shareware stuff like goldwave for free to do it with.
My idea was to have a look at the xditties.ast file which is 110mb and I'm guessing the songs would be in there somewhere one after the other with gaps of silence in between. It all hinges on being able to open or convert those files hopefully there is a way.