I've been looking for this since I first realized that you could find old network bumpers on RU-vid!! I remember the airing well (I saved the TV Guide issue for all the feature film TV premieres that interested me in the 1970s... sorry to say I dumped them in one of my many moves... :( )
The Sound of Music Debuted for the first time on TV February 29 1976 on the ABC Network's Sunday Night Movie. The AC Nielsen Rating was a whopping 33.6.