How do I download a Facebook messenger audio file?
UPDATE: The mobile website solution pointed out by others doesn't seem to work anymore. If you can get it to work, please update this answer again. For now, fallback to one of the methods described below.

UPDATE: As pointed out by others, you can open Facebooks' Official Mobile Messager Webpage and download directly from the interface:

Go to https://m.facebook.com/messages/
Find the audio file you want to download
Click on the dropdown menu on the right of the audio file
Press Download



These generic methods should work for most browsers. You just have to watch out for the fact that different browsers will use slightly different names for the same tools. I will outline the general solution bellow and indicating the exact tool names for Safari, Chrome, and Firefox. (Feel free to edit this answer to add the names for other browsers!)
Method 1 (Network)

Go to the chat window that contains the audio file you want to download, using the Facebook's Messenger Official Website.

Open the browser's Web Inspector

All Browsers the keyboard shortcut opt+cmd+I,
Safari: Develop -&gt; Show Web Inspector,
Chrome: View -&gt; Developer -&gt; Developer Tools,
Firefox: Tools -&gt; Web Developer -&gt; Toggle Tools;


Go back to the chat and press the &quot;Play&quot; button of the file you want to download.

Find the Network tab in the Web Inspector and you will notice that an audioclip-xxxxxxxxxxxxx-xxxx.aac file has been loaded.

Double click on the file and it should automatically download to your default downloads folder (e.g. ~/Downloads). Alternatively, you should be able to right-click on the file and choose the &quot;Download&quot; option.


Method 2 (Resources)

Same as Method 1.

Same as Method 1.

Same as Method 1.

Find the Resources (see details bellow) tab in the Web Inspector and find the audioclip-xxxxxxxxxxxxx-xxxx.aac file

Safari:

Tab: Resources
Section: All Resources
Folder: Other


Chrome:

Tab name: Application
Section: Frames
Folder name: Media


Firefox:

Please add here




Same as Method 1.