I already figured that were music plays there must be a source.
Since the mobile home simulate a house interior it would make sense that the RV had a internal stereo device, so I checked it.
Edit: That would be a exception for locations like the Town Hall and Re-Tail which don't have a stereo of course.
I saw the horn in the Nintendo direct, I found out about the stereo by accident when I was going to honk the horn and I accidentally pressed A before I got on the chair.