Here is a collection of reviews of Steamboat Willie.
https://drgrobsanimationreview.com/2010/05/01/steamboat-willie/
http://lauraview.blogspot.co.uk/2012/01/steamboat-willie-analysis.html
https://magicalmoviereviews.wordpress.com/tag/steamboat-willie/

These reviews mention that Steamboat Willie was in fact not the first cartoon to use synchronised sound but a cartoon made in 1928 named Dinner Time was the first cartoon to feature synchronised sound. There is also a mention of Fletcher Studios, which experimented with sound in 1924.