I have a two channel audio output device for PC. For some reason one of them is broken down. So, I have logically an one channel audio output device. Can I still use it to hear sound outputs belonged to all frequencies from PC by shorting left and right channel wires and feeding it to the working speaker? The sound quality may be degraded that way.
Yes you can, the stereo signal is just two fast changing voltages and a ground signal and it's totally industry standard plug compatibility for mono stereo jacks to fit suitable speakers.
You can add the two voltages because they have mostly have the same phases of amplitude for most music, with dual mic recordings logically you can get some small phase cancellation effects in certain bass notes if the mics were positioned so that a particular wavelength arrives with inversed phase at either microphone, it's practically unnoticeable, especially in higher frequencies.
Alot of music is made stereo on a mixing desk in so there won't be much phase cancellation.
You will lose the stereo information but all the sound information will still be in the mono signal.