What happens after someone listens to my mote voice note message? Can they delete it?

Your messages can't be deleted by anyone else. 

The only person who can delete your messages is you - do this from your 'My Activity' page

Google Classroom Private Comments are great for leaving permanent feedback records for students.

Google Docs, Slides & Sheets comments can be 'resolved', but they remain accessible by accessing the 'Comment history' of the document - see below.