This morning I happened across an MGM short from 1930 on TCM: "Song Writers' Revue." Jack, as MC, introduced a number of well-known songwriters from the era, including Gus Edwards ("School Days, School Days"), Nacio Herb Brown, Arthur Freed, and five or six others. Each played a snippet of one of his famous songs, and at the end, Jack asked them to put a lyric he had "written" to music (it was a melange of cliches). They do so, and the song becomes the closing number.
Jack was in good, though early, form in the role, making sarcastic, snide remarks as he introduced each songwriter. Traces of his later mannerisms--the crossed arms, the fingers to cheek--show up. However, this early Jack is just not as funny as he became later on, once he realized that he functioned better as the recipient of sarcasm rather than the disher-out of sarcasm. I don't know if the short is in rotation on TCM, but you might want to check the listings for it.