To download a voice message from Facebook Messenger, the most reliable method is to use a web browser on a desktop or laptop computer by following these steps:

1. Open a web browser and go to messenger.com. Log into your account.
2. Find the conversation with the voice message you want to download.
3. Right-click on the voice message audio player and select "Inspect" or use the browser's developer tools.
4. In the developer tools, go to the "Network" tab and filter by "Media".
5. Play the voice message so it loads in the "Network" tab.
6. Locate the media file corresponding to the voice message, right-click it, and select "Open in new tab" or "Download".
7. Save the audio file to your computer.

Alternatively, you can use m.facebook.com (the mobile version on desktop browser) and click the three dots next to the voice message to find a "Download" option. If access to a desktop is not possible, a workaround on mobile devices is to use the built-in screen recorder to capture the voice message audio. There are also Chrome extensions available that can facilitate downloading voice messages from Messenger and Instagram chats with a click. This method works on both Windows and macOS desktops using Chrome or Chrome- based browsers for best results.