Shortly after the Warner Bros. animation division recovered from a six-month closure in 1953, nearly every major movie studio switched from recording their movie/short soundtracks on optical film to recording them on high-fidelity magnetic film. Warner’s scoring stage was retooled for stereo recording around this time, and most (or some?) of their “Looney Tunes” and “Merrie Melodies” were recorded on this stage in this manner from 1954 to circa 1963(?).
This thread on the Steve Hoffman Music Forums chronicles Hoffman’s job as a music storage archivist at Warner (among the things he remembers salvaging are some hi-fi “LT”/“MM” scores). Although most of the material he was working with was thrown out shortly after his time there, it is implied a few pages into the thread that he transferred the scores to digital audio tapes. He also recalls finding a magnetic stereo track for What’s Opera, Doc? (much to the animation department’s delight), indicating that at least a few of the short scores were indeed recorded stereophonically.
However, all of the isolated music tracks for the magnetically recorded shorts on the Golden Collection DVD sets and the Platinum Collection Blu-rays are in mono, and (AFAIK) the packaging and liner notes of both volumes of The Carl Stalling Project CDs don’t say if anything is in mono or stereo. However, some shorts have aired on Cartoon Network with stereo soundtracks before, including (according to a Golden Age Cartoons forum thread post) Bonanza Bunny. Are there any other stereo soundtracks to the post-shutdown WB shorts making the rounds (like on The Carl Stalling Project CDs(?), hi-fi VHS/digital recordings of the CN broadcasts, etc.)? It would be interesting to see and hear the 1990s-2010s restored versions with whatever is known to exist (or circulate).