Hello, since some users of the umc1820 and ada8200 have the same problem (alsa is muting some channels)
i have found out a quick solution:
open a terminal window and type in the following:
alsamixer
press F6 to select sound card (in this case UMC1820)
The displayed sound-cards are starting with 0 (that means the first one has the value zero, the second one the value one and so on)
Find out the right value for your umc1820
Exit alsamixer by pressing ESC
type in the following lines: (the value after the “-c” parameter has to be changed to your sound-card number that you just discovered in alsamixer. In this case change the “2” to your own number)
amixer -c 2 cset numid=1 on,on,on,on,on,on,on,on,on,on,on,on,on,on,on,on
amixer -c 2 cset numid=3 127,127,127,127,127,127,127,127,127,127,127,127,127,127,127,127
amixer -c 2 cset numid=9 on,on,on,on,on,on,on,on,on,on,on,on,on,on,on,on
amixer -c 2 cset numid=11 127,127,127,127,127,127,127,127,127,127,127,127,127,127,127,127
amixer -c 2 cset numid=8 0
alsactl store
The lines will enable all outputs, enable all inputs, set all output volumes to max, set all input volumes to max and set the clock to internal .
that last command will store your configuration.
this did the job for me without reinstalling the whole os.
do it on your own risk!