Here is a list of categories that might be useful to you. I'll explain my own organizational methods after the list:
Evil Music
Pleasant Music
Tribal Music
Exotic Music
Underwater Music
Volcanic Music
Creepy Music
Battle Music
Tavern Music Exotic
Tavern Music Mellow
Tavern Music Upbeat
City Music
Cave Music
Forest Music
Sewer Music
Desert Music
Random Music
My music is categorized according to my planar game. I listen to music, then decide which of the planes the song relates to the most. Then I place that song in a playlist for that particular plane. For example, Beastlands music, or Mechanus music.
I'll try to break down my planar categories into more generic categories for you based on the mood of the plane:
Evil planes = Evil Music (usually creepy but "harder" sounding music)
Good planes = Pleasant Music (soft & relaxing music)
Exotic planes = Tribal Music (Native American & African music with lots of drum beats & flutes) & Exotic Music (usually Indian, Asian, or Arabic music)
Elemental planes = Underwater Music (my air, water, & even Astral plane music is similar, so I'd just call it Underwater. Earth & Fire music could maybe be lumped together as Volcanic Music or just use Evil Music if you ran a Volcanic scenario)
Ethereal plane = Creepy Music (sort of like evil music, but more soft & spooky "Halloween" style music)
That should cover the basics as far as how my planar music could be separated into more generic categories.
Then I have other categories for more generic moments:
Battle Music - fast paced music that gets the blood pumping
Tavern Music - music I can imagine a bard playing, separated into Tavern Music Exotic for exotic bars, Tavern Music Mellow for relaxing taverns, and Tavern Music Upbeat for the party tavern.
City Music - Elven City Music, Evil City Music, & any specific city music (like Sigil)
Cave Music - creepy & very slow music
Forest Music - peaceful traveling music
Sewer Music - similar to cave music, could probably just use Cave Music for this (I have a lot of songs, so I have enough for both categories)
Desert Music - arabic type music, could probably just use Exotic Music for this.
Random Music - any song I liked but didn't really get me in a mood that would make me relate it to another category. I play Random Music when I get tired of hearing music from another category, or when the moment in game is nothing special.
BTW, I tried using RPG Deck a while back. It would have been great to use, but all of the features the creator mentioned that he did not include are all features that I need in order to use RPG Deck. Not being able to skip around the songs in a playlist sucks, since I like to skip songs I'm tired of hearing or if I want to start the playlist at a certain song. Being able to play songs from a playlist in random rather than from first to last would be nice. Starting and stopping a sound effect would be nice since some of my effects are an hour long (birds chirping, rain falling etc etc). I also couldn't figure out how to remove a soundtrack from the interface once I added it. There were a few other features I needed, but for a free program, it's still nice.
I just can't seem to find any program that provides all the features I need in order to easily run background music. I've even tried DJ software. My best solution is to use Winamp & Windows Media Player at the same time, and that's not very convenient.