Is there a way to generate and cache audio waveform previews?
In some browser's yes...
User Manager, username, WebInterface section, customizations section, "Audio inline player : Show waveform". I don't know if IE supports this...its using HTML5 to do it.
Ah right, it's a client side preview so it has to be downloaded before we can see the preview. Any way a preview can be generated server-side?
If you have a command line tool that will generate a JPG of an audio file...then yes. But I don't know of any tools for that myself. If you have one, it could be configured in the Preview tab in the preferences to handle MP3 and other files.