See if you have an automute setting in alsamixer
In a terminal window
alsamixer -c 0 -V all
There you will see the slider controls for various things, like Master, PCM, Front, etc...
You may need to use the right arrow to scroll horizontally until you get to see everything..
Look for a setting called "Automute"
The up/down arrow will toggle that setting on or off.. Sounds like you wish it to be set On