The Steam Deck disables the internal microphone when using the 3.5mm jack to connect a headset. The Steam Deck expects any attached headset to have a microphone and will disable its internal microphone when one is connected. As of right now, there is no way to forcefully enable the internal microphone. This issue is not present when using Bluetooth or USB-C to connect headsets; the internal microphone should still be available with those headsets.