Nope, this is still not possible. Based on what the developers have said in the past, it seems like it will probably never be added.

Which is unfortunate, because... Gotta tell it straight here; the SRR vanilla music is pretty sweet, but I'm not sure if I could handle playing more than a couple UGCs filled with that same setlist of tracks. I can see myself getting tired of the monotony and... sensory underload, fairly quickly.

Harebrained Schemes: highly recommend you guys allow adding of custom audio. I've worked on a few mods, and let me tell you, while visuals matter, the soundtrack is often the emotional soul of a game that keeps players jacked in and "feeling the vibe." I think half of the positive and ecstatic comments I got on my Deus Ex mod were simply, "DAMN, AWESOME SOUNDTRACK! WHERE CAN I BUY IT!?" Just a little goes a long way.

There was an interesting thread on the forums trying to figure out how the soundtrack is packaged in the local files. In the end, all we know is how the music files are packaged (sequential OGG files merged into the file "resources.assets.resS") and that you can theoretically rename/repackage other songs to replace your original in-game music. AFAIK, however, there isn't anyone that has successfully managed to figure out how this benefits adding custom music to any UGC.

This would be awesome to see happen, i mean it may not be easy to do. But generally if you bang your head against something for long enough you can figure out a way. It was done with the save system. If there was enough demand for it then maby, i believe anything is possible

I can't see it being terribly difficult to allow users to add in music. It's just a matter of swapping out an audio file. Simpler than portraits, really, which have dimensions and such to deal with.

must all be related to legal issues, copyrights and blah blah, they dont want to be sued.

Actually, they've stated a few times that it's more of an issue with how Unity3D handles the audio assets. However, when all is said and done, they've never really seemed that interested in creating a workaround for adding the ability to import custom music into the editor.

The aforementioned quotes:
From the thread "Adding Music in the Editor"

引用自 Brian_HBS：

No, music is tied in much too deeply to Unity's sound management to easily add custom tracks.
That said, I wouldn't be shockied if a clever modder figures it out, but it's not something directly supported by the editor.

From the thread "Ambient sounds would make the game 100 times better "

引用自 Brian_HBS：

There's currently no way to mod in sounds or music -- those elements are too deeply crunched and optimized deep inside Unity during the build process, so it wont gracefully allow new sounds to just slip into a content pack (the way that new portraits, props or items can be).