A - Some Noise is common on any switching type of power supply due to the nature of the power conversion. As long as there is switching of this type, there will be noise generated. Each of our lights has a switching power supply in it, as is the case with many electronics today.
First; separate the power wire running to the lights from any other wiring on vessel. When the power lines touch (are in same conduit, zip tied together, bundled) it can cause transference that is causing the speaker noise. Run across/perpendicular to bundles. 9 out of 10 times this eliminates the noise.
Secondly; you may install a relay between the lights and the shortest distance to the power supply, then connect your switch to the relay. Separate all power wires to the lights from any other wiring on the vessel. The Relay should be rated for approximately 20% more than the total amp draw of the lights being installed. The circuit should be fused properly based on approximately 20% more than the total amp draw of the lights (in-line, fused bus bar, fused circuit breaker are all acceptable depending on you installation).
Lastly, or in addition to the above; try putting a 4700 µF Electrolytic Capacitor across the positive and negative power terminal at the back of the amplifier (or radio). As long as you get one that is 35V or higher, and over 4700 µF, it ought to do the trick. The capacitor has a polarity marking (stripe with - symbol goes to the negative terminal) so be sure not to connect it backwards.