They distributed their processed music-like product on tape, recorded
at an especially slow speed, at one time and the tapes went back for
erase and reuse or destruction at some point. They also distributed it
via subcarrier on FM and maybe in some market still do. Either way a
few nutty eccentrics intercepted the PMLP and retained it for their
use. Many "ambient" musicians such as Brian Eno are known to possess
some quantity of Muzak PMLP and to have studied it att length.

A few "secret" LPs were made also for demo and inhouse training
purposes. More than one has escaped.

If you have a time machine and Muzak is your first concern of
interception, you are one sorry dude. It's like a '62 Reissue Strat in
Seafoam Green...