If you are talking about using SWA files - the Macromedia Shockwave audio format - they are at their core basically MP3 files in disguise; they both conform to the MPEG1 Layer3 specification. Therefore the compression/sound quality ratio for SWA is identical to that in MP3.
As such, if you are needing stereo hi-fi music quality, you'll probably want to use 128K for minimum CD quality, or go higher up to 192K to improve the sound, but pay the cost of higher bandwidth.
If you don't need the fidelity, or - as is often the case with Flash multimedia - you need to keep the SWF loading time to a minimum, then drop the bit rates down to 96 or even 64K and keep them mono.
HTH,
G.