Download Voice Message from Messenger (Browser)
Open Messenger in your browser and go to the chat.
Open Developer Tools
Windows/Linux: Ctrl + Shift + I
Mac: Cmd + Option + I
Go to the Network tab.
Filter by Media (or search audio).
Play the voice message in Messenger.
Look for a file named audioclip or .m4a / blob entry.
Right-click → Open in new tab (or Copy URL and open).
Click Download.