Came across a couple of posts on Google about how to do this, so I can actually get songs onto my iPod. It seems like it’s purposely made impossible to figure out by yourself.
1. Make sure you’ve got gst-plugins-lame installed.
2. Read this and set up a new sound profile. Make sure you check the “Active” box.
Alternately, some random J5 guy recently posted about MP3 for free from Fluendo.