1)Most basic is just burn it to a CD or put it on an ipod and let it run in the background. This has worked for years, requires minimal intervention during the game, and works just fine.
2) A slightly more complicated approach is to set up different groups of tracks or playlists of tracks that share a theme - In Town, In a Tavern, Battle Music, Forest Music, Dungeon Music, etc. When your players hit a new location, switch to that playlist. This is pretty easy to do and prevents the aural disconnect that happens when Anvil of Crom kicks in while buying potions at the healer temple. This is a good compromise between prep work and payoff. It's easy to update too - if the party heads to sea, add a "Nautical Music" folder, probably flavored with a lot of Pirates of the Caribbean. Over time you will build up a nice library of themed music and the players will pick up on it when they hear familiar tracks.
3) Most complicated is the full suite of music for your campaign by location, character, activity, etc. This is a lot of work and I have only done it once. It was worth it but if you are pressed for time don't bother with it - go with #1 or 2 above. In mine, each character had a theme, each step in the adventure path had a theme, each town had a theme, the taverns they frequented had different themes, eventually their ship had it's own music, each dungeon had separate music - it's a lot of work but at the time I loved the way it turned out. The players liked it, but probably not as much as I did. I switched playlists depending on where they were and had several different sets of battle music - one for shipboard combat, one for underground combat, one for fighting in the city streets- it was overkill, really, and I probably won't go that nuts again. Plus, with 8 players for most sessions, I already had a lot to do and didn't need the extra overhead.
Let's get to specifics now since I am running 3 campaigns at the moment:
- Basic D&D - my players in this one are new and haven't really heard any of this stuff before so I'm going to keep it simple. I haven't been running any music for them yet but as we get back on a more regular schedule I will probably bring in some combat music using the classics - Conan, Starship Troopers, Gladiator - maybe some LOTR but I'm leaning away from that right now. I will see how they like the battle music and go from there. Pounding Poledouris is more their speed I suspect.
- Necessary Evil - I haven't been using music in this one, but I want to. Some of the NFL Music may be recycled here. The Robocop soundtrack is good and underused in my games so I may use it as a major theme. I'd like to have a track for each character and some general background music and just let it roll while we play - no switching between folders, etc. Big heroic themes are good so this will mostly be movie music.
- D&D4 - this one I will be spending a little time on. Since Phlan is divided into districts, I am thinking about having background music for each district so I don't really have to worry about switching during most of the game. Civilized Phlan will basically be "Town" music while the slums will have low-key slinky music like some of Aliens and Escape from New York. The Graveyard will have spooky stuff. Sokol Keep will have sad "after the battle" type music while the Temple of Bane will have big blaring martial themes. I will probably also do a Main Theme for the campaign and a track or two for each character, just because I like that kind of thing. I would play that at the beginning of each session while were getting ready and then go to whatever section of the adventure we are in after that - give it an "opening credits" feel for each session.
A note here: I don't think any of this is strictly necessary. I mainly do it because I like it and it makes a campaign feel more polished or complete to me. As long as the players don't hate it and I like it then it's a net plus to the game as it helps me get in the right frame of mind to run. If you're the DM and aren't that interested in the musical aspect then don't sweat it. You might consider farming it out to a player who is though, if you have one. Tell them you'd like some music for the game but you don't have time or whatever and let them run with it. My players cringe when I ask for backstory but for NE I asked for character portraits. Since we all play City of Heroes and it has an awesome character creator, they had fun designing a character for a totally different game for a change and the results were very nice - I will post them here at some point. The point was that now everyone has a character illustration regardless of artistic talent and is that much more invested in both character and campaign. If your group has someone with musical inclinations let them do some of the work here and you will have similar results- more atmosphere in the game and more investment by at least one player.
Finally a technical note. We used to play in my living room so I could use a burned CD or DVD with all of the tracks on it playing through the big surround system and operate it with a remote. This worked well but now with a much smaller group and in a different house we tend to play in the kitchen at a table. The solution to this is the ipod/iphone in a small speaker dock right by the table. It's quite audible but doesn't overwhelm the table conversations. It's easily controlled, has useful audio apps for impromptu musical needs, holds more than CD's and even DVD's typically do, and can be organized in a very detailed way with playlists that are easily modified from session to session. I will be putting it to the test with my campaigns and will note how it works out down the road.
That's the end of the spic RPG Music discussion. Hopefully it will be useful to someone else out there. I didn't realize beforehand that I had quite that much to say but I have used it a lot over the years and I think it does add a lot to this hobby of ours.