Recently the school district where I work had a F.I.D (flexible instructional day) and a number of teachers used the Mote extension to leave audio feedback or directions to their students (click here for a blog post I created about Mote).
The issue with Mote is that while you have 20 free uses a month, they are a business and need to make money. Plus, they have a number of additional options. Click here for their pricing options.
The site Beep has an extension that allows you to leave voice notes for free.
Once you go through the sign-up process, make sure you refresh the page you are on (like a doc, sheet, etc). You will then see Beep appear in the upper right of your screen. Simply select and then record - please note it does start as soon as you hear the - you guessed it - Beep.