To change the volume anywhere in Sibelius 6 you can add a midi message into the score.
Normally the hidden messages are not displayed. To view press the VIEW button, Hidden objects.
Midi controller 7 determines the volume the tilde squiggle ~ is the start of a midi instruction.
~C7,0 is soft ~C7,127 the loudest
Create text or use the quick keys CTRL+ALT+T
Enter ~C7,127 at the desired note or staff.