Sony BRAVIA KDL-26T30 Series Operating Instructions Manual page 24

Speaker
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"Sound Effect", "Treble", "Bass", "Balance" and "Auto Volume" are not available when "Speaker" is set to "Audio System".
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GB
Turns on/off the TV's internal speakers.
"TV Speaker": The TV speakers are turned on in order to listen to the TV's sound
through the TV speakers.
"Audio System": The TV speakers are turned off in order to listen to the TV's sound
only through your external audio equipment connected to the audio output sockets.
When connected with an HDMI control compatible equipment, you can turn the
connected equipment on interlocked with the TV. This setting must be made after
connecting the equipment.