I use a lot of music for my games as I think that a proper atmosphere can be attained, and involvement can be a lot more intense if the right theme is playing. I try to avoid specific soundtracks unless I'm gaming in the world that that soundtrack was written for, such as Howard's 'Lord of the Rings' music or Williams' 'Star Wars' soundtracks.
For example, using the Star Wars themes for a setting that isn't Star Wars immediately takes away from game immersion; the players won't be thinking about the Alien-inspired dark horror gunfest you've got designed, they'll be thinking about lightsabres and starship dogfights. It's all about making the music fit the game.
The thing is, I've been running out of soundtracks. Using certain music for a game results in that game being equated with the music, so using it for another setting or campaign doesn't help. So how else can I have background noise and maintain a unique atmosphere every campaign?
Youtube to the rescue.
I've recently discovered background noise on Youtube that can sometimes go on for hours. Simply go to the search bar, type in the type of sound you wish and then follow it with the word 'sound'. Changing the length of the video to 20 minutes and over will get you some very long effects so you can just leave them on and forget about them.
I've found pretty much everything I need up to yet; the sound of a river running, the sea, a forest, a jungle, night sounds, engine noises for starships, aeroplanes and trains from varying ages, thunderstorms, city noise, wind; all kinds of mood-enhancing background noise can be found on Youtube and it covers pretty much everything you can imagine. Even if you can't find exactly what you're looking for there's bound to be something that closely emulates it.
There's a lot of stuff out there, some better than others, so I've built up a library of sounds in my favourites in my browser so that each one is just a click away. I can also set them up in different tabs so they're right there when I need them, it takes just a few seconds to switch them on when the players move from one location to another. It's good for laptops, tablets and smartphones so there's plenty of ways you can get hold of the sounds you want, no matter where you are.
So the next time you're thinking of a way to enhance the atmosphere, give Youtube a try. It works for me.