I've been taking a look at Syrinscape, a sound mixing/playback app aimed at the tabletop roleplaying market. The app is free, and there are some free soundscapes available with it, but for the most part the soundscapes cost about four bucks a pop (and there are some collections available which will get you a small price cut for buying a bunch of them at once).
It assumes, of course, that one has one's gaming area set up for sound, and that one is able to make use of a computer (or iOS/Android tablet) at the table. That's not a big ask these days I guess.
I like the way that it blends sounds together, and fades seamlessly from one to another. I've always fancied the idea of being able to have ambient sounds running during the game — things like wind noises, rain, thunder, crowd/street noises, all that sort of thing — to reinforce mood, and this app seems to be ideal for that.
Unfortunately my gaming area isn't set up to make best use of it right now. My computer is nearby, but I'd have to be getting up and down to activate the noises I want, which is far from ideal. However, I do have a little netbook that sits around mostly doing nothing, and I have a spare set of speakers, so I might just give it a try-out one of these days.