How do I delete a mote voice comment?

Before or after you've shared a mote, you can delete a voice note completely from your Activity page - just go to www.mote.com/account/activity

However, if you just want to delete a mote that you've created, before you share / send it, here's how to do this in various different applications:

In Mote comments within Google Docs, Sheets and Slides

First click on the three dots in the top right of any comment you create. 

Then just select Delete.

In Google Classroom Private Comments

In Google Classroom, unfortunately you can't delete a private comment from within the Document view window. Instead, you'll need to go out to the individual student 'Student work' view, (in the 'Student work' view, just click on the student's name in the left pane), and click the three dots over on the far right of the page, see bottom right in the picture below:

A similar approach can be used for motes shared within the Class Comments or Stream views.