Hey, magenta4793591! I tried to walk through these steps to understand better where you're bumping into trouble. I get to this section:
and I click the blue link that says +Add Custom Greeting.
Then I get this pop-up:
and so I click Choose File.
I don't actually have any MP3 files on my work laptop, so I had to stop there -- but can I ask, once you select an MP3 file from your computer/laptop, are you not seeing the magenta Save button in the bottom right corner? Or do you click it, but then nothing happens?
Lastly, do you have the ticket number you received? While we aren't able to file or escalate tickets from this public user forum, we can check status so I'm happy to take a look at that for you if you like!
Marissa posted some awesome steps and screenshot. Can you take a peek and see if that helps you out? Thanks!