LG 47LM9600 Owner's Manual page 197

Sound
HOME Settings
Try Sound Test first to verify that sound signal output is normal.
When the test sound is normal, check external device connections or broadcast
signals.
Problem
Screen display is on but no
sound.
For analog broadcast, stereo
is not clear or sound comes
from one speaker only.
SUPPORT Sound Test
Check other TV channels.
Make sure the
In a poor reception area (areas with weak or unstable
signal) or if multi audio is not stable, select
Multi Audio.
Solution
TV Speaker is set to On.
Mono on