Split into three audio channels, music, ambience & SFX, you can set any kind of mood you want! Designed specifically for roleplay events, but usable for anything. Need the atmosphere to be rainy, but you're in sunny Mulgore? Just activate some rain sounds and you'll be just a little bit more immersed. The raid marker over there is supposed to be a Vrykul? Play some Vrykul sounds on their actions to really drive it home.

Installing Shortwave is enough to let you listen along to sounds your group leader plays, no further configuration required.

Each audio channel has a seperate broadcasting & listening setting, so that you can listen to your own playlists whilst letting your DM set the mood, or quickly search through sounds without broadcasting them to your group.
Just listening requires no configuration whatsoever.
Extensive searching through all audio.
Always using in-game music? The addon turns your active music off for you, and back on whenever you're done.

Some known issues/difficulties with the WoW sound API:

Pausing is not possible, you can only start/stop sound files and WoW only tells you if they're finished or not, now how far in you are.
Muting your sound or turning your master volume to 0 will mess with playback and instantly stop any song you're listening to.
All of the search data has been split into seperate addons so you can disable them as needed. Have a low memory system? Considering turning off the SFXData addon in the AddOns menu. Blizzard added a LOT of creature sounds.
Some sounds won't play, or won't have audio. This is usually just on the game, e.g. music from SoD shows up in the list, but won't play. Some voiceovers will be encrypted, and won't play unless you've recently gotten the game to give you the key. Nothing to be done about these, and no way to detect which is which other than experience.

Shortwave Audio Player