Sound input devices detected but record no external input

I recently built a Gentoo desktop system for someone else and they've noticed that sound input devices (microphones and headsets), while they are detected and registered correctly, do not detect any input apart from the static (if jostled in the jack). This occurs regardless of whether the front panel microphone jack or the motherboard's built-in jack is used. I confirmed that the problem exists as described and also double checked using an Ubuntu live CD to ensure that I didn't make a mistake in the Gentoo setup process to cause it; the issue is indeed experienced on Ubuntu, so I'm ruling out a distro-specific issue. Because the issue occurs with the front and back panels, I also don't believe an audio header connection issue is the problem (plus, I checked and the audio front panel connector was plugged into the motherboard correctly with no bent pins), and it's not a problem with any of the microphones since they work fine with other computers.

My guess is that the built-in sound card is defective on the input side, or some other hardware defect. That's the only cause that I can think of, but I wanted to make sure that nobody thinks it's something else, as the person for whom I built the PC has a low budget for additional components/return shipping. Thoughts?

CPU is Intel i3-10300, motherboard is MSI B150M PRO-VD, kernel version is 5.12.x-gentoo. System logs available on request but I don't have immediate access to the machine so I can't post them in the OP at the moment.