Hi Toril S,
You can 'queue' the next MIDI song for playback when one is already playing by simply selecting the song you want to play next from the Song Selection display. If you select a song, 'NEXT' appears at the upper right of the corresponding Song name. To cancel this setting, press the 'NEXT CANCEL' (lower #7) button (make sure that 'MENU 1' is shown at the bottom right corner of the display).
Alternatively if you want to play a number of songs one after another, then you need to set the Repeat Mode accordingly.
You do his by pressing FUNCTION > Tab to MENU 1 > SONG SETTING > Tab to OTHERS
Here are the options shown for 'Repeat Mode' on page 55 of English version of the 'PSR-S970 Reference Manual'
REPEAT MODE: Determines the method of repeat playback.
OFF: Plays through the selected Song, then stops.
SINGLE: Plays through the selected Song repeatedly.
ALL: Continues playback through all the Songs in the specified folder
repeatedly.
RANDOM: Continues playback at random through all the Songs in the
specified folder repeatedly.
Regards
Ian