I want to test my Nextcloud instance to host/keep my audio files for museums, walking tours.
Any pointers on how to go about it: eg where to keep the audio files, how to get the rss feeds, which apps I can activate etc.
If you use MP3-files you can install the app “Audio Player” to play MP3 in the browser. But in museums and on walking tours you do not need the app 
https://apps.nextcloud.com/apps/audioplayer
Your solution: Also you can sync your nextcloud-files (or folders with MP3) to your mobile device (Android, iOS) and hear the MP3-files offline.
Sorry. No idea. I think you must download the MP3-file from the rss feed and reupload it to your Nextcloud.