I have been using wpaudio before WP3 and shared a lot of the experiences aired in this thread. And now I seem to have ironed out my troubles. So I would like to share my findings with you (as of WP3.4.1 + WPAudio 3.1):
1) You should make the change in the minified jQuery-code as mentioned in these threads
They are more less pointing to the same solution, but in this:
mrienstra has an interesting poing that the code might be case sensitive so an '.MP3'-extension will be ignored. I have not tested it. But good point.
And in this flomincucci points out that she needed to do the changes in "both js files (wpaudio.js and wpaudio.min.js)". Interesting point.
2) If you're still having trouble with the download button?!
Here's what I found:
The documentation mentioned that dl="0" will suppress the download link:
[wpaudio url="/thepath/soundfile.mp3" text="the_title" dl="0"]
Subsequently, I falsely assumed that dl="1" would activate the download function. But this turned out to disable my download link instead. So you simply have to leave it out an rely on the default behaviour which includes the default button.
Hope this will cut a corner for some.