Sending an audio message in response to an email can be useful when you need to use some voice inflection to deliver meaning that you might not be able to deliver with just text. This is particularly true when replying to students who are still developing their reading skills or when replying to parents whose first language isn't your own.
In the following videos I demonstrate how to record video and audio messages directly from your Gmail inbox.
Two Ways to Create Videos in Your Gmail Inbox
How to Record and Send Voice Notes in Gmail