It seems that the keycodes for the C3100(possibly C1k/C3k) Ok/Cancel buttons are different:
Cancel is 102, Ok is 92
So my xmms.xmodmap looks like this:
//
keycode 102 = C
keycode 92 = B
keycode 133 = Up
keycode 134 = Down
//
i.e cancel is play/pause, ok is forward, up/down is volume up/down