Sony AIBO ERA-301P2 Mode D'emploi page 27

Setting the volume level
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Press the MODE/NEXT button repeatedly until the volume level
indication appears.
Press the SET/CANCEL button.
The current volume level setting starts to flash.
Use the DISPLAY/– or LIGHT/+ buttons to change the volume
level. Then press the MODE/NEXT button.
The setting is entered and the volume level indication resumes. The volume
level can be set to four levels. If you select "0," AIBO's musical tone is
muted.
https://www.usersmanualguide.com/
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