I don't think you are doing anything wrong.
You can try this: Go to the powerpress control panel and then Settings > Basic, for the Episode Entry box setting enable the Quicktime video player cover image option. You need 1.0.3 (latest version) to access it.
It also depends on what version of quicktime the user has. It might work, it might not. Give it a try.
The mp3 flash players are just for mp3. The only way to use the flash players (audio only) is to use mp3 files.
Hope this helps!