I have this same problem. I don't have a solution, but I have at least some more insight for whoever might be able to help debug this. This problem persists no matter how I encode the MP3 (192 or 160), with metadata in the file or not, no matter which browser - and from multiple computers.
The thing that makes me think this is an Apple problem is the error I can find, and reproduce. In Safari, in the Develop menu, I can open the Web Inspector. I switch over to the Network tab within there, and go through the usual upload process. As the upload happens, I can scroll lower in the network activity showing there. Lots of successful connections are made to various domains (including some that if you click on, show the web address of the new file on Apple's server, with a GUID-based name). However, when the upload "finishes", each time there is an error (highlighted in red) at the bottom of that list of successful connections. You can click on it, and for me it shows this JSON response:
{
"type": "UNKNOWN_PODMC_EXCEPTION",
"locKey": "ITC.PCC.ServerErrors.UNKNOWN_ERROR"
}
Any amount of searching shows me this is a configuration issue on a server, and not because I've done something wrong. But I'd be happy to be wrong. I can only hope for some additional assistance on this that doesn't involve the basics of re-encoding the file.