That is right, called SCA for sub carrier authorization, I used to make adapters to hear what was on this band. Several FM stations had not Muzak, but their own “business music” to compete.
Then someone found more money in using the SCA to deliver commodity futures quotes which sounded like a 300 baud modem but was scrambled.
There was also a doctors bribery service called Physicians Radio News, it told doctors about new drugs to prescribe, and offered TV sets, free CD’s and even some free vacations for writing enough prescriptions so I listened to that one more than Musak. Another thing was some PBS stations had reading service for the blind and would read novels and such so much each day.
By the way this SCA sounded poor, you could always hear some interference from the main FM channel, one reason it was always played at a low level. Musak later moved to little sat dishes, and then last I heard it went under as it just was not needed anymore. A DVD full of business music could last all day now.