Synthesizer missing

Headphones are plugged in and the sound from youtube and media player is OK.

I found the instructions about I/O in the Handbook and tried it a few times, however without any result, most probably because the API and the Device fields were empty. I also restarted the computer several times, as my knowledge about computers basically stops there, but all these futile attempts and my own ignorance made me so frustrated that I just shut down the whole thing and went out for shopping. When I came back after about 2 hours, I started the computer again with no expectations whatsoever, and behold, the Playback was black, so was the Synthesizer, and in the I/O the API field shows MME and the Device shows Microsoft Sound Mapper - Output. I guess it is just as it should be, because now the sound is working normally. However, all this took me a whole day, I started about 10am, and now in my time-zone it is over 10pm. Of course, this whole story is far beyond my understanding, but maybe there are some experts out there who can find a reasonable explanation, as even in my deep ignorance I refuse to believe that going out for shopping can be the solution. I really appreciate that you kindly tried to help me, also on behalf of others who may encounter similar difficulties.