that is what media player is for. you can run it while running CoH. all you have to do is turn down the music and special effects for the game and you can hear it. or get an mp3/ipod and listen to it while playing.
oh and just to add, that would be changing the games programming which is a BIG no no. you can get your account banned for it.
CoH Splasher already can do this for you.
http://www.splasher.com.ar/
"Mastermind Pets operate...differently, and aren't as easily fixed. Especially the Bruiser. I want to take him out behind the woodshed and pull an "old yeller" on him at times." - Castle
There actually is a way to do this without manually messing with any game files: Leandro's Splasher program. With it, you can also change the pocket D music as well as the music for the hero and villain login screens. This is all in addition to being able to change various in-game textures, such as the login screen pictures, with the program. Now, you'll just be limited to what you replace each boombox track with, but it's better than nothing. If you want different music always playing, do like Quint suggested and have a media player of some sort running while you're in-game.
Edit: Curse you, Oldeb!
Positron: "There are no bugs [in City of Heroes], just varying degrees of features."
It's a really nice idea, and I can tell it's got wishful thinking and lots of good intentions, but tell me something; what happens the first time someone of severe racial prejudice logs on and sets their custom music to his own mantra of 'I HATE N WORDS!' Let's say that sentence was modified to be a bit more offensive, like including said 'n word'. Let's replace it with 'I HATE KITES'... While I love kites and feel everyone is entitled to their opinion, a T becomes a K, and I'm not too happy to see someone insulting my ethnicity.
Sigium, I don't think the OP was suggesting that everyone hears his music selection, merely that he can change his music (doable by dumping OGGs in the appropriate data folder, or using Splasher).
What you're fighting agiainst wouldn't happen anyway, regardless of potential abuse. If the game were altered such that everyone can hear your music selection, that means you've got to download their song(s), and that they have to upload them. Depending on the file, this could take a while...
http://www.fimfiction.net/story/36641/My-Little-Exalt
This is doable already, even without the splasher program. (Though that'd be easier). The folder you need is (COH install folder)/data/Sound/Ogg/Boombox/ and the filenames are TrackX_loop.ogg where X is a number from 1 to 25.
Note that'll replace the existing boombox music. (and only that, the rest of the game is unaffected and the only one who'll hear your custom track is you, everyone else hears default music) Though, I don't know anyone who actually likes the default boombox music so meh.
As a sidenote, you can change all sorts of other things in there too. Like having the ghost ship play the Ghostbusters theme, or having the propaganda speakers on the Council Empire map play Hell March.
[Admin] Emperor Marcus Cole: STOP!
[Admin] Emperor Marcus Cole: WAIT ONE SECOND!
[Admin] Emperor Marcus Cole: WHAT IS A SEAGULL DOING ON MY THRONE!?!?
I'm just asking for like a folder somewhere inside the CoH folder where you can put music in, and in game under the emotes will be Tracklist/ Boombox - Custom and the names of music you put in the folder.
Ex.(Folder)
C/Progam Files/City of Heroes/(Account Name)/Custom Music/(File: Coming Undone)
Ex.(Music)
Emotes>Music>Tracklist>Boombox - Custom>Coming Undone
I just suggested putting it under the account name folder thinking it would help with only being avalable to you in gameplay.
Please respond.