Who did a Voice over for Mickey Mouse in 1 early cartoon while working for Disney? Yet he was more famous as a composer working for Disneys Rivals from 1936 till the late 50's

I believe that would be Carl Stalling

Wouldn't happen to know the title SR?
I looked through the IMDB list and it is listed Disney throughout

The Encyclopedia of Disney Shorts says it was "The Karnival Kid", released in July 31, 1929. One user comments in the Internet Movie Data Base comments: "Mickey's voice was much different than it is today. He had a much more squeaky voice in this and in several shorts to come." This one featured Mickey's first words, "Hot Dogs!"

This site says that "Wild Waves" was one of a few that Stallings did Mickey's voice. This one is also from 1929.

Here it says "Klondike Kid" which I suspect was meant to be "Karnival Kid" since it states Mickey says "Hot Dogs!: in it, and in the plot of "Karnival Kid" , Mickey was at a hot dog stand. Note also that "The Klondike Kid" was made released in 1932, after Stalling had left Disney (1930).
"Disney.com Offers Vintage Mickey Mouse Short (Movies) posted: 2/21/03
Visit Disney.com to view Mickey Mouse in Klondike Kid. This 1929 short features Mickey's first words, "hot dog" According to the Encyclopedia of Animated Shorts website, Carl Stalling, the short's Musical Director provided the voice of Mickey." - laughingplace.com (Bold mine - DG)