I haven't tried Wimpy mp3 so I can't compare.
But from using WordPress and playing mp3's on some of my sites (I have not tried selling them), here are some ideas.
My basic concept is: First be able to put a playable & downloadable mp3 on your website. Second, make it so that only paid people can access the full download (let others stream the song anyway?).
For the paid people -- you could either make it so they pay-per-song or per-download, OR you could have them pay to Register for your site, like a subscription. Then give Registered people access to the mp3 downloads, and hide the download links from Non-Registered.
Use or try any of these 4 methods of playing an mp3 on your site.
PodPress may have some options about "Premium content", I'm not sure, look into it.
The second step depends on your intended method.
If you want to sell each song individually -- you should probably look at Shopping Cart methods, plugins, services.
If you want to sell Registration on your wordpress site, I think there is a plugin that helps with this (couldn't find it right off, but it's out there) (??)
If you want to then show part of your Posts (like the Download links) to Registered, and Hide that part of the post from Unregistered, there is a plugin for that or you can code that by hand in your theme templates.