The short is one of the standard "music and dance" toons that were so common in the early days. Mickey decides to put on a talent show of sorts featuring a live orchestra of barnyard animals.
Like I said, my favorite part about this cartoon is its animation. Mickey decides to sing his theme song, "Minnie's Yoo-Hoo", and the animation paired along with it is simply the strangest of Mickey I have ever seen. What with the wacky movements, bizarre poses, and just plain grotesque facial expressions, it's a rare chance to see Mickey act completely out of character.
As for the song itself, "Minnie's Yoo-Hoo" was the melody played over the title cards of almost all of Mickey's early shorts. The song was the very first piece of sheet music released by Disney in 1930, and went on to be a hit. Once the 1950s came along, Mickey received a new theme song with the all-popular "Mickey Mouse Club March". For those interested, the lyrics of the song can be found here.
Minnie appears in her own, literal box seat to sing the last line of the song to Mickey, "yoo-hoo". Now technically Minnie said this word in Steamboat Willie, though it was more of a squeal that sounded like "yoo-hoo". She for sure said "who me" in Plane Crazy, so Mickey's Follies marks her 3rd (or possibly only 2nd) speaking line.
Besides Mickey's burst into song and Minnie's first word, Mickey's Follies is a pretty uneventful toon. I thought I would mention another design mistake that appears on a mouse that isn't Mickey. Not only does this mouse bear Mickey's shorts, but it's actually the size of a real mouse. That's a little strange considering Mickey is his regular size in this toon, making him seem like a giant compared to this smaller creature.