But how does Muzak get to Mom'n'Pop's Store?

It's hitching a ride on a covert channel: The SCA subcarriers on the regular FM band.
Subcarriers need a baseband frequency on which to ride. So in a manner of speaking, Muzak() really is an evil parasite leeching bandwith from, say, your favorite NPR station. This method of transmission is being phased out (pun!) in favor of digital satellite channels.
Oh and by the way, don't try to demodulate that subcarrier, it's illegal.