My basic rule is No Singing! I want orchestral movie them type music for most of my RP sessions so that's what I look for. I do make some exceptions occasionally, mainly when it's a language no one at the table understands well enough to translate on the fly, but most of what I want is just music - no singing.
Movie soundtracks are source #1 for me. The big advantage is that they are usually well done and widely available. The downside is that they have to be appropriate - Star Wars games need Star Wars music, but in a Traveller Free Trader campaign they are wildly out of place. If I was running 50 Fathoms for Savage Worlds or a 7th Sea game then the Pirates of the Caribbean would be my core material - 3 movies worth of nautical pirate music is a no-brainer. However, this can be a double-edged sword. If one of your players listens to this every day on his way in to work, he may be bored and jaded with it - it's appropriate but worn-out. Some good Rp-useable movie soundtracks that hopefully aren't worn out for your group:
- Fantasy - Gladiator, Last of the Mohicans, Legends of the Fall, Braveheart, the Recent BBC Robin Hood TV series, and Aliens.
- Science Fiction - Babylon 5 has several soundtracks out there and they should be less recognizable for many groups then Trek or Star Wars. For more gritty or cyberpunk type games Escape from New York is one I used in a Shadowrun campaign and liked.
- Western - look for Ennio Morricone soundtracks and you are bound to find something you will like. Themes from various western TV shows can be handy too.
- Special - The Godzilla soundtracks have some good marches and martial themed tracks and usually a few quieter ones too. If your player's aren't fans they probably won't recognize them either.
- Supers - this is a tricky one because most of the music from superhero movies is pretty closely associated with that particular character or group. They can be co-opted if you're doing say an homage - our old Professor Y and the Y-Men campaign could have used the X-Men music pretty easily - but it's tricky. You may be better off with another source. For one of my characters that originated in City of Heroes, Adamantium Man, I use the Fox NFL theme as his personal theme music - if you've ever listened to their pregame show you know the tune. It sounds bold and dramatic and while it's familiar to some it doesn't make you think of some other super hero.
Classical music pieces can work well - many gamers past a certain age will know the end music from Excalibur, for example. Ride of the Valkyries is another well-known example. I am not a great guide here as I haven't used a ton of them but selected pieces can fit in quite well with other music if you stick to the orchestral / no vocals idea.
New Age/Techno/Space Rock/Electronica - Again I am not a great guide here but every decade seems to have it's instrumental concept music fad and these may fit well with your campaign concept, and if you don't want the orchestral movie feel this is a good place to start. Yes/Pink Floyd/Tangerine Dream all have instrumental tracks or ones with minimal vocals that might work in some campaigns. Cyberpunk games need some club music at some point and some kind of electronic or techno can give them a very different feel than a sweeping instrumental score - online searches are your best friend here as that's where you will find most electronic music, even moreso than other music. The flavor here depends on whether you want more piano, strings, or synth in your background music.
Finally, my best source other than movie scores is...computer and video game soundtracks. To some it may be old hat but if you haven't used them before they are a gold mine of good stuff waiting to be used. Some suggestions:
- MMORPG's often have soundtracks available for free download somewhere on their sites - you may have to look but they are there. Two I know of for sure - City of Heroes, Dungeons and Dragons Online. Others may not have company downloads but often have files that can be located or extracted from the game files if you have installed them at some point like Everquest 1 & 2, World of Warcraft, and Lord of the Rings Online. These are very handy and usually have a lot of location-specific tracks - tavern music, dungeon music, battle music, etc.
- CRPG's usually have decent soundtracks and some special editions of them come with a music CD containing some of the game's best music. I personally have CD's from Ultima, Icewind Dale, and Titan Quest among others, and all have seen use in my games. Baldur's Gate has a few music tracks, as do Morrowind and Oblivion.
- Strategy Games - One of the best soundtracks of all time is the music from Total Annihilation. if you don't have it go find a cheap copy online and copy those tracks- it's very good orchestral stuff that sounds good in any genre. Heroes of Might and Magic 2 also had some nice orchestral tracks if you can find a copy. The Dawn of War games have some good tracks that could fit multiple genres.
- Shooters - Halo music could fit some campaigns. Various updates of the Doom soundtrack might have a place too. Most of them tend to be louder and faster and more guitar/rock/industrial than I want in a fantasy game but for a cyberpunk or Mechwarrior game they could be just fine.
- Wild Cards - the NFL soundtrack I mentioned in the last post has a ton of useful stuff and doesn't fit into one of these categories. Music from the Olympics or other sporting events could be very useful if you can find it. Military performances may have some good marching or memorial music that can be used in parts of your game.
- Internet Radio - Having a iphone or ipod or a computer with Pandora Radio on it can be useful - I needed background music for the ball in the Freeport Trilogy and used Pandora to pull up some Baroque chamber music that sounded appropriate and the players thought it was great. I'm not going to buy a bunch of tracks for that kind of thing but having a source of on-the-fly music for special situations is very handy. If you need period music for a game like a 1920's Call of Cthulu session then this may be a good way to fill it in.
There are most of my sources - tomorrow some thoughts on execution.