Hi yukskywalker,
Thanks for visiting the Apple Support Communities. It sounds like a specific function of the Facebook app isn't working as you expect it to. If the audio issue is isolated to the Facebook app, we recommend troubleshooting the app itself. It sounds like you've tried doing that by uninstalling and reinstalling the app. This is a good step to take, and is actually part of the Apple Support troubleshooting for an app-specific issue. Please ensure you have completed all of these recommended steps next, which you can find here: If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open - Apple Support and below:
1. Restart your device
Restart your iPhone or restart your iPad. Then open the app to see if it works as expected.
2. Check for updates
To see if updates are available for the app, open the App Store, tap your profile icon, then scroll down to see if updates are available. If updates aren't available for the app, update the software on your iPhone or iPad.
If all of these steps have been completed and the issue persists, please get in touch with Facebook support directly. As the developer of the app, they will be best equipped to address issues that occur with the functionality of the app. You can get more information on how to contact them here: How to contact an app developer - Apple Support
Cheers!