On the instruction manual, here is a possible cue:
"If you hear a consistent “hiss” or hum, try
turning UP the volume of your TV or other audio
source, and then set the ZVOX volume level
LOWER. If your TV is connected to a cable box
or satellite receiver, try turning its volume control
UP – so that when you switch from your cable
box to your DVD player, they play at about the
same level. As a rule of thumb, try setting each
device at about 75-90% of its maximum volume
level -- then turn the ZVOX 325’s volume control
up as far as it will go without distorting.
In some circumstances, you will hear excess
hiss if your TV/audio source is turned up TOO
high. If this happens, try adjusting the TV/audio
source sound level DOWN and turn the ZVOX
325 volume UP."
I adjusted my TV volume to zero previously...oh! well.