Hi Pincho,
this sound (a Bass plus a Pad sound, when you play a chord with the left hand while ACMP is switched ON) comes from the "Stop Accompaniment" function. You cannot really change this sound, but you have three options (go to "Menu > Style Setting > Stop ACMP"):
Off: No Sound is heard when playing a chord with ACMP on and the Style is stopped.
Fixed: The same sound is heard for all the Styles.
Style: The Bass and Pad sounds of the current Style are used.
Please refer also to page 12 of the Genos Reference Manual.
By the way: This setting is automatically memorized to Registrations when the Style checkbox is ticked.
Hope this helps!
Best regards,
Chris