Samsung UE40ES7000U E-Manual page 746

ΠIf you are using an external device,
check the device's audio output
option (ex. you may need to change
your cable box's audio option
to HDMI when you have a HDMI
connected to your TV).
ΠIf you are using a DVI to HDMI cable,
a separate audio cable is required.
ΠIf your TV has a headphone jack,
make sure there is nothing plugged
into it.