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The Muzak can be heard everywhere in the building, not just on the elevators.
Technically, it's illegal to play copyrighted music without paying royalties to record companies, even if it is to keep people calm and entertained on an elevator or while holding on phone lines; Muzak is a less expensive alternative.
It also (if it doesn't feature any singer) prevents people from becoming upset by offensive lyrics, or the company looking "unprofessional" for playing such.