I have a PSR S770 and yes all my midis are in one folder.
Here's what it says in your manual (p 67):
You can play multiple Songs repeatedly:
[FUNCTION] TAB [E] MENU 1
[H] SONG SETTING TAB [F] OTHERS [G] REPEAT MODE.
So, go to Menu 1>Song>Repeat Mode, and set to Random.
Fred