I only recall strongly the final years of "The Young Sound" on the old WEEI-FM-103.3, and it seemed to me to be pretty much an automated top-40 sound with Dick Provost as the Boston "voice" of the format (I think he also did a weekend talk shift at the old WEEI-590 at the same time).
It aired on CBS's other FM stations, and I suspect each city had their own local host doing an early version of "voice tracking" for the format in that city.
Around 1972, it was replaced on the old EEI-FM by a partly-live "soft rock" format, while at WCBS-101.1 in New York, the now-legendary oldies/classic hits format replaced "The Young Sound".
Also in the very early seventies, I recall that the all-news 5-9 A.M. "AM Report" was simulcast on both AM and FM (at the time, the AM had news blocks from 5 to 9 A.M. and 6 to 7:30 P.M. on weekdays and talk the rest of the time), with the "Young Sound" starting at 9. I don't think the FM broadcast 24 hours a day until it went "soft rock", so "Young Sound" may have run from 9 A.M. to 12 Midnight, 1, or 2 A.M.