Check out this interesting history of MGM article and happily, Jeanette and Nelson are mentioned. The website itself is quite interesting and informative:
It’s 1928, and the success of Warner Bros’ musical, The Jazz Singer, has ushered in a new age of talking pictures. Audiences adored it, and it was sink or swim time for MGM. Suddenly, the silent cinema rule book was thrown out of the window and numerous opportunities opened up in Hollywood.
Composers were in demand, and song and script writers, along with voice coaches, were needed more than ever. White Shadows In The South Seas was the first MGM sound picture, although not a talkie. Originally filmed as a silent picture, MGM realised that sound wasn’t just a passing fad and, like most studios at the time, swiftly added sound effects to its music. But they did make one character speak – and that was Leo the lion, who roared for the first time.
The first MGM talkie picture, and the first MGM star to speak on the screen, was William Haines in crime drama Alias Jimmy Valentine. The film was only part talkie, but it was nevertheless a step in the right direction for MGM. The new technology meant a big change around for the studio – for some stars, their career was over, and for others it was just beginning. Stars like Clark Gable, Jean Harlow, Robert Montgomery, Myrna Loy, Jeanette MacDonald, and Nelson Eddy were among them.