When inserting Line in, Figure 1 is grounded.
The Q2 S and D poles are conductive.
The chip U7 Pin 3 VCC connects battery voltage 3.7V.
Meanwhile, Pin 11 SEL1 is also elevated through resistance R16.
Vbus is suspended, so U7 pin7 SEL2 is low. U7 switch to Audio mode.
From U7 pin8 and pin10 plus 20-20KHz, 100mV sweep signal, pin6 and pin12 have very high distortion, up to 30%.
