I'm afraid we're not aware of a good solution to this.
One peculiarity of Google Classroom is that comments you add to a Google Doc or Slides template are not replicated when you create a unique copy of the document for each student.
And if you share a document with comment view enabled, so that all students can see the comments, then any student can resolve the comment.
Here's an article from Google's help site confirming the lack of a solution:
UPDATE - SOLUTION FOR GOOGLE SLIDES!
Check out mote for Google Slides as a great solution for this use case.
For Google Docs, here's a couple of options:
1. Use the motepad to record your voice note, and then paste it into the body of your document.
2. Use the motepad to record your voice note, and then paste it into each copy of the document, in the comment field.
Don't know how to use motepad? ⬇️