Because the pins have to be pulled high the opto would ideally have to have an PNP transistor in them which is not normal. Yes it might be possible to use an NPN in emitter follower mode, but it might not switch high enough.
Maybe COM is the more positive point, put your black meter probe on COM, red on PLAY, what is the voltage AND polarity?
i dont really know how to measure that properly,
Are you sure that you are not using a X10 probe?