Hi Dermot J,
Unlike earlier PSR models, the S970/770 has a function similar to that on Tyros models which controls what happens when you plug headphones into the keyboard.
Press Function > Utility (in Menu 1) > Config2.
Select Option 1 - Speaker. This is the setting that controls the behaviour of the on board speakers.
At the bottom of the display you will see a list of possible settings for this.
The options are:
HEADPHONE SW: Speaker sounds normally, but is cut off when headphones are connected to the [PHONES] jack.
ON: Speaker sound is always on, even if headphones are connected.
OFF: Speaker sound is off. You can only hear the instrument sound via the headphones or an external device connected to the AUX OUT jacks.
For auto switching of the speakers when you plug in headphones you need to set this to HEADPHONE SW.
This is a system setting, so once you set it, it should stay as set until you change it, or carry out a system reset after which it will revert to the default setting (I don't know what this is on an S970/770, but I would have thought that it would be SPEAKER SW)!
Regards
Ian