Murena cloud : sync some things (recorder audio notes) and not others (music)

Via the default Account Manager / eDrive, no. To do that install Nextcloud app for finer control or some other tool, like Syncthing, FolderSync, et al.

I haven’t tested in a long time but in the past even the Documents directory was synced as part of the ‘Pictures and videos’ part. Not good if you wanted to just sync the former and not the latter.

Use any third party tool and you should be good.

Install and connect Nextcloud app to your /e/cloud drive to unveil its true potential - eOSWiki / HOWTOs

Don’t forget the last post by @petefoth for some updated info.