I was told last year or the year before, that it was “against” the rules to play music and/or listen to headphones. I didn’t bother to look it up but I certainly don’t see the harm in having a speaker playing if it’s not bothering everyone else.
I use my speaker all the time at home. I think as long as the foursome doesn’t have a problem with it, then what’s the offense?? Can it actually give someone an advantage?