The canonical way to include sound files in LaTeX beamer is to use the \movie command of the multimedia package. This also works in pympress, however, it replaces the text link (or whatever is used) with a black box while the audio is playing. See the attached minimal working example for a .tex, .ogg and .pdf file displaying that behavior: test.zip

This PR is a straightforward fix:

• In the GstOverlay, define a media_type attribute that is overridden during construction (which calls set_file). I followed the original code to create a class attribute media_type that is later shadowed by an instance attribute. (Which may or may not have been intentional, see https://www.bruceeckel.com/2022/05/11/misunderstanding-python-class-attributes/. It's also not done consistently, e.g., sink does not have a corresponding class attribute. I'd also be fine removing the class attribute and setting an instance attribute in the constructor instead.)
• In set_file, ask Gio to guess the mime type from the file extension. This works fine as long as the extension is sane. If a user has a video file with an audio extension, it will be mistaken for an audio file, but I guess we could live with that.
• In on_play, only unhide the video sink if the mime type does not start with audio. This is more defensive than only showing the video sink if the mime type starts with video, so it should not break things when mime type guessing did not work.

Let me know if you'd like to see any changes! And thank you for maintaining pympress, I think it solves all the issues I had with pdfpc or evince 🎉