Here are some guidelines on how to do this using your Firefox browser:
How to post voice messages via messaging
Part 1: Preparations
1. Prepare your settings
Click on the messages link on the left-hand block in your course.
2. Select settings
Click on Settings
3. Check html editor and non-frame settings!
Part 2: Recording a message
1. Write a caption
If your browser plays up and you can't write a title for the sound clip, use this workaround: write the caption into the editor, then select and right-mouse click to copy to clipboard.
2. Paste caption
Right-mouse click in caption field to paste caption text.
3. Record, check recording and send.
Record (see volume meter = green bars), then stop, play back to check before clicking the OK button below to insert the sound clip into the message.
Note: you can send several sound clips in one message if you want.