Owner's manual says it effects the 'tone' :

Linear adjustment (LINEAR)
The tone values for the set signal source (e.g., radio/
FM) are set to “0” (mean value). Press and hold the
AUD button for more than 4 seconds until FM LINEAR
appears (which affects FM only). Press and hold
the AUD button for more than eight seconds until ALL
LINEAR appears (which affects both radio and CD).