I suspect that the sound that you describe is the so called 'Stop Accompaniment' (STOP ACMP).
You can select one of three settings for Stop Acmp on the 'Style Setting' display, accessed by pressing [FUNCTION] > TAB to MENU 1 > [G] STYLE SETTING. Press [C] to select STOP ACMP and change the setting using the #1 or #2 buttons.
Here is what page 27 of the 'PSR-S975 Reference Manual' says about this function:
When [ACMP] is turned on and [SYNC START] is off, you can play chords in the chord section of the keyboard with the Style stopped, and
still hear the accompaniment chord. In this condition—called “Stop Accompaniment”—any valid chord fingerings are recognized and the chord root/type are shown in the display. Here, you can determine whether the chord played in the chord section will sound or not in the Stop Accompaniment status.
OFF: The chord played in the chord section will not sound.
STYLE: The chord played in the chord section will sound via the Voices
for the Pad channel and the Bass channel of the selected Style.
FIXED: The chord played in the chord section will sound via the specified
Voice, regardless of the selected Style.
If you select 'STYLE' for STOP ACMP, then the voices currently in the Bass and Pad channels of the loaded style will sound. You can change these using the Mixing Console (STYLE) display, and you can adjust the volume of the two channels, but of course any changes will be also heard if you actually start the style. You would need to save the style with any such changes if you wanted to recall if with your modifications.
You can't change the instruments that sound in STOP ACMP if you select 'FIXED' as the setting.
Regards
Ian