Hi Roberto,
1. Thank you for this question and you feedback about the documentation, I have added a link to an external page listing audio support in HTML5. You surely can use WAV files, which actually appear to be compatible with the most browsers.
2. There currently is no alternative to using your own servers, since download takes place automatically in the background (as opposed to manually, at the initiative of the participant) which requires the servers where the ZIP file is hosted to grant the domain where the experiment is hosted (e.g. spellout.net) such automatic distant access to your ZIP file (e.g. using a .htaccess file, as described in the documentation).
2′. I am sorry if the documentation is unclear: one cannot refer to GDrive/Dropbox folders, but one can directly refer to GDrive/Dropbox files directly (and inidividually) as long as they have a public-access URL. With Dropbox, you can click “Copy Link” next to the file you want to use and replace the ?dl=0 bit at the end with ?raw=1 and use the URL in a newImage, newAudio or newVideo command.
Let me know if you have further questions or requests.
Jeremy
