I realized that I had actually made a few of the Micropub.rocks tests more strict than they should have been. There were a few tests about uploading photos, when really the thing they were testing was how to upload files of any sort. The spec even mentions video and audio explicitly, so the test suite should accept those as well.
Today I updated the test suite to allow video and audio uploads for test 104, 203, and 300.
Marty was quick to test his podcast client Screech with it, so his client now passes test 300! When an audio file is uploaded to the test suite, the post preview shows an audio player with the uploaded file.