Free GM Resource: Syrinscape
Syrinscape is a pre-arranged soundboard of RPG music and sound effects. You can download the program for free and it comes with two Soundsets: Bugbear Battle and Witchwood. You can purchase more Soundsets for $4 a piece or the whole current lot for $20.
Now I have some minor quibbles about this particular product, namely that the website mentions that "Unlike other soundboard-style apps, Syrinscape uses editable and adaptable libraries of sound files". These files are editable or adaptable only in the sense that you can turn them on and off or change the volume, but this is for a one-time use only. The program doesn't remember any changes you made and you cannot save your settings in any way.
Softrope, but I know there are a lot of
GMs who really want something simple and would like to incorporate sound-effects into their game. Syrinscape is definitely simple to use and when the Android & iOS versions come out I think it'll add a lot to some GM's game.
Right now there are two free Soundsets to play with and by toggling on and off the various effects I think you can get a lot of good free usage from Syrinscape while you evaluate it.