I'm on approach.  Gear Up.  Flaps 5, Flaps 10.  So far so good.  Throttle is minimal.

 

I extend flaps to 15 and hear a horn.  I've tried a black button on the overhead panel (lower right) and also the right hand side of the throttle quadrant without any luck.

 

When I extend the gear or increase N1 to over 45% the horn stops.

 

Is there a way to silence the horn ?

I don't think it's possible to silence the horn once the flaps are at 15. SOP is flaps 1, 5, gear down/ flaps 15, landing flaps. The intermediate flap settings like 10 are there if you need them, but from my understanding are typically skipped.