Bluetooth adapter messed up built-in line out function

[verycd]

I plugged in a bluetooth adapter (receiver and broadcaster in one) to my MInt PC today. It works fine with bluetooth earphones, however, lineout stopped working properly. Namely, instead of sending signals to the amplifier and speakers, when lineout is chosen, it sends signals to the bluetooth earphones. When the BT adapter is removed, there are three options left, HDMI output (monitor), lineout built-in audio and dummy output. But there is no sound when lineout built-in aduio is selected. In the volume control app (pavucontrol) > configuration, analogue stereo output, digital (HDMI3) stereo output and analogue duplex are available but none of them makes lineout work.

Being lazy touble-shooting, I tried to use the 3.5mm jack on the monitor, i.e. the HDMI output, instead of the line out, but audio is not synchronized with video on youtube. The speakers built in the monitor just suck. Therefore, I have to post this issue here for help. Thank you.