I’m a huge fan of using sound in RIAs. I think that having audio cues is just as important as visual cues and that a click sound, or a subtle noise when you interact with an application makes for a much more usable experience. Unfortunately it can be kind of a pain to add those kinds of sounds to Flash applications. In general you have to go hunt down royalty free sounds and use MP3 files in your application which can add a bit more size to an application than some people want. We made some changes to the sound APIs in Flash Player 10 that let you generate sounds and while I was in Singapore I talked to the guys from Sonoflash who have a really great solution.
You can download a series of libraries from them which are all different soundscapes and have different themes. Add one or more of those SWC files to your application and then you can just call the sounds with some simple ActionScript code. All of the sounds are generated by AS3 an so take up a lot less weight than having to deal with MP3s. It also means you can tweak the sounds on the fly and use the APIs to modify the pitch, frequency, or other variables when you call it. I did a video with them in which they show off some examples and how to use it.