I was having an issue with my new Steam Deck where when I plugged in headphones the internal mic would disable (specifically in Discord). With multiple hours of searching, I was able to find that this is a known issue, and there is no fix for it. I found 2 temporary fixes. I will outline the 2 fixes here. Both methods require that headphones are plugged in before proceeding
Method 1:
Switch to desktop after plugging in headphones.
Right click on speaker in taskbar.
Click on 3 bars next to microphone/input.
Switch to internal microphone.
Boot back into gaming mode.
Should work until headphones are unplugged and replugged in.
Method 2(preferred):
Switch to desktop.
Open start/taskbar menu.
Scroll down to multimedia.
Right click on PulseAudio Volume Control.
Add to steam.
Reboot back into gaming mode.
When plugging in headphones, you can now navigate to NON-STEAM in library and select PulseAudio Volume Control.
Change control layout to mouse.
Navigate to Input Devices.
Scroll until you see ACP/ACP3X/ACP6x Audio Coprocessor Headset Microphone +Internal Microphone.
Click on drop down next to Port.
Switch port to Internal Microphone.
Exit application using Steam button (for some reason steam+B does not work).
Done!
This should work by navigating to Library from any application when plugging in headphones.
Enjoy!