update: i tried hooking up an aux cable to record using line in, and it  can only record the right channel. i can also tell that this problem is not only with headphones.

using a Line In device

Hello. Is the device a usb audio interface, or something else?

I'm surprised you are able to make that work, because the combo jack is a headset jack. But you said you have done it before, on the same laptop?

So there are multiple symptoms...

Recording -- will only record one channel through the combo jack, when earlier it was recording both.

Playback -- audio only plays through one channel of the headphone jack, but both channels come through the  laptop's internal speakers? I assume that testing was done with various sources.

I can't think of much other than a configuration setting or hardware failure. You removed and re-installed the Realtek audio driver, which would have normalized the audio configuration settings, so my guess is that the combo jack has suffered some damage.

When you say aux cable, does it have a stereo plug (TRS)

or a headset plug (TRRS)?