The stuttering is SLC noise. The high pitched tone could be the amplifier supervision tone or could be something else. You may need to run TSP for the speakers. However, there are some things that can be tried before that.
What kind of wire is used on the SLC? If it is TSP, where is the shield landed?
You can try making the negative side of the power supplies in the two panels common together. If the SLC shield is landed on chassis ground move it to power supply negative. The noise situation could get better, get worse, or stay the same.
On connecting background music, keep in mind that in fire alarm equipment the power supply negative is not common to chassis ground. That is for the required ground fault sensing. Typical audio equipment has the power supply negative common with chassis ground. You may need a 1to1 audio isolation transformer between the equipment to isolate the grounding difference. Those are available from various sources including car stereo retailers.