*READ*
There is probably a way to control the volume with the HDTV remote.
THIS is from your own manual:
"When using the audio cable connection
After making a connection, press the HOME button, then select [Settings] — [Sound] — [Headphone/Audio out] — [Audio out (Fixed)].
Turn on the connected audio system, then adjust the volume.
Hint
When using an external audio system, the audio out of the TV can be operated using the TV’s remote control if [Headphone/Audio out] is set to [Audio out (Variable)]. Press the HOME button, then select [Settings] — [Sound] — [Headphone/Audio out] — [Audio out (Variable)].
When connecting a subwoofer, press the HOME button, then select [Settings] — [Sound] — [Headphone/Audio out] — [Subwoofer]."
So..
If I read this correctly, you may need to change from "FIXED" to "VARIABLE".
I think "fixed" is for people who use an alternative remote (like home theater) to handle volume. If that works, also choose an appropriate volume using the physical speaker knob. You may need it close to 100% to be loud enough, but it depends how strong the input is.