LG OM4560 Service Manual page 30

NO SOUND
There is no sound output in the USB FUNCTION, repair the set according to the following guide.
5-3. IN THE USB FUNCTION
5-3-1. Solution
Please check and replace IC502 on MAIN board & CN801 on FRONT board.
5-3-2. How to troubleshoot (Countermeasure)
1) Check +5V_USB to FRONT board.
  If an insert, search, or file search on the VFD display USB Device into the USB jack, the voltage is okay,
if so not, check +5V to pin10 of CN801.
2) Check USB D± from MAIN board to FRONT board.
a. Check 1.1_D± signals(pin A7, A8 ) to IC502.
b. Check USB± signals to CN801(pin1, 2, 4, 5).
  If there is any trouble, check the power for each IC.
The power is normal but , if the signal waveform to the IC is distorted or no signal, replace it with a new one.
3) Check if "Digital audio AMP block" on item 5-2 is normal.
5-3-3. Service hint (Any picture/ Remark)
ONE POINT REPAIR GUIDE
3-13
< USB function signal fl ow >
< Waveform of USB D± signal >