No microphone audio when calling over Signal or doing microphone tests in Waterfox

A friend of mine uses Linux Mint as their operating system. But since a while now they're unable to get the microphone audio to work. It's detected by the operating system and applications, but no audio is detected by the application or heard by participants, except for Discord.

I wasn't able to find the cause of this issue, we've tried common solutions like rebooting, removing the device from the Bluetooth device lists (they use headphones), etc.

Any help would be very appreciated :)