I have been fortunate enough to play in multiple sessions that a friend who has studied sound design had the time to actually make sound effects and background sound files for. It wasn't a huge thing that took the stage front and center. No, he poured hours of work into a long, multi-hour subtle sound effect. Stressed metal, dripping pipes, creaks and groans of shifting weight. All sounds you hear in an old, crashed hulk of a space ship. No one thought it was doing anything for the game. A half hour later nothing in particular had happened, but everyone at the table was tense and hanging on every word in the scene. It was an incredible experience.
Unfortunately for me, I'm not a sound engineer nor am I particularly versed in sound design. It's just not in my skillset, and I don't have the hours and hours of time needed to study it (nor even an inkling of where to begin. However, we do live in an age of the internet and this hobby of ours has a lot of creative people into it.
Which is how I found this site: tabletopaudio.com. I say I found it, but in actuallity that friend who did the sound design for his games pointed it out to me. I've been clicking through over the past couple hours and it has a lot of good stuff in it. As I'm writing this post I'm listening to a "song + ambience" for a super hero game. It's pretty good. I've also listened to some of the pure ambience files, such as the colliseum, black smiths, and a few other places. You can even queue up audio files to play in order, or just skip around while searching via genre.
Give it a look. Maybe some of the sound is just what you need for your next session, or to make the introduction of that big bad guy all the more menacing. Oh, and below you'll find the video where the folks at Bioware talk about the story I told above.