Sony AIBO ERS-31L Operating Instructions Manual page 40

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Slide the VOLUME switch to adjust the volume level, then replace
the cover.
To increase the volume level, slide the switch towards the + position; to
decrease the volume level, slide it towards the – position. When you release
the switch, it returns to the center position.
There are four volume levels, including mute. Each time you slide the switch
the level increases or decreases by one level.
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