So I have no idea how to proceed. There's a command in the Advanced menu called Create MP3 Version, but it's grayed-out with the cursor on the memo.
Update: I found a way to do it. I had to dig around in the Finder to find the file (they also removed the Reveal in Finder command). I copied it to the Desktop, then dragged it onto the iTunes icon. Boom. There's the Convert to MP3 command. So they just took it out for Voice Memos. (I can hear their internal discussion. Very few people use it so let's move it. I happen to be one of the people who use it.)