It turns out, amazon offers a really nice stock interface for playing MP3s. But it's hidden. Here is what to do:
- Make a new collection on your Kindle. Call it "Music" or something similar.
- Connect your Kindle to your computer.
- Open the "audible" folder, and create a new folder inside of it. It doesn't matter what the new folder is named.
- Take your music MP3s, and drag them into the new folder.
- Disconnect your kindle. The new MP3s should appear on your home screen.
- Add them all to the "Music" collection you made on your Kindle earlier.
Simple as that. Now, you can go to the "Music" collection and you get a nice list of the MP3s in the folder. If they are tagged with artists etc, those are shown too.