And why would the music from the MP3 stop when I hit the horn
I MAY have a clue. I just encountered the same thing last night so I did some testing. Here's the setup. I have Bluetooth adapter in the trunk with the iPod connector plugged into it. I have two 12 volt to USB power adapters in the space under the headlights. The USB power adapter has an on/off switch so I can turn it off when not needed. One of the adapters goes to the phone to keep it charged while I'm listening to Pandora. The phone is connected to the BT adapter in the trunk. I also have the CB communications system and am listening via wired headset.
When the switch to the USB power adapters is off, and the cord is plugged into the phone, and the phone is playing Pandora via the BT adapter (AUX on the screen) every time I press the horn button the audio mutes while the horn is blowing and for a couple of seconds after. I checked and the audio will mute for every audio source. If the USB power adapter switch is on, no muting. This is what I encountered last night.
I can't see the connection between the switched off USB charge cable being plugged into the phone and the horn blowing. The only electrical tie in between the two, when the USB adapter switch is off, is the ground for my air horn compressor and for the USB power adapter both go the auxiliary fuse panel I have.
I just did some more testing before I posted this. The horn mutes all audio when the USB power adapter is switched off. The charger cable does not have to be plugged into the phone, and the BT receiver nor the iPod have to plugged into the aux audio port in the trunk, i.e., I unplugged the iPod adapter cable. The bike has to be running. The muting does not occur when the cluster is powered up by holding in the mute/eco button when the key is turned off. The USB switch has to be off when the bike is started. If the switch is on when the bike is started and then turned off, the muting does not occur.
The speakers also mute. I checked with external speakers on and off, and the headset in off or mix mode.
Weird is all I can say at this time. There must be some sort of glitch being generated in the CAN protocol somehow. It does provide me the benefit that if I'm using the phone to listen to Pandora, and blow the horn, and the audio mutes, I know the power to keep the phone charged is switched off, and the phone will die in about a 1/2 hour or so.