Unfortunately I’m not aware of other people experiencing the same issue. Taken together, the facts that playback works just fine and that a ZIP file does get uploaded to the server are really puzzling: there’s no indication of a problem on the participant’s end nor on the server’s end…
I just did a test run myself: can you check on your servers whether the mp3 file of my recording is also silent? You should hear 3 (rather unpleasant) whistles
Jeremy