Streaming audio, especially flash, is not suitable to use for downloading if people are purchasing music.
I would suggest to use streaming audio in flash format (such as MP3 Soundstream) to provide short samples of the music on your website. This will provide good quality, even at dial-up speeds, while being reasonable protection against theft of the music (although there is not much incentive to steal a short sample).
Then, when the music has been purchased, you will need to provide a secure link to an MP3 file of the music to download. The link needs to be password protected and available for a limited time.
Rick