How to get mp3 to play from browser instead of downloading (or how to open another file type in browser)
Finally figured it out; and since earlier posts raised the issue but didn't have this solution, I decided to post what worked for me.
1. Download an example of the file type you're having issues with 2. CTL-J or click the down-pointing arrow icon ("display the progress of ongoing downloads") at top right of menu bar 3. Right-click the file name of the item you just downloaded; then choose "Always open similar files"
Adding the file type to Settings > Applications was not an option. Windows Media Pack was a red herring. Autoplay is a different thing.
Hope this helps someone, or maybe ME the next time I forget.