The Pixar short film shown before Inside Out in theaters was Lava.
Directed by James Ford Murphy and produced by Andrea Warren, Lava is a musical short about Uku, a lonely underwater volcano who sings for millions of years while hoping to find a mate. A second volcano, Lele, hears his song, creating a romantic story told almost entirely through music.
Lava premiered at the Hiroshima International Animation Festival in 2014 before receiving its major theatrical release with Inside Out on June 19, 2015. Its title song was also released as a digital single and appeared as a bonus track on the Inside Out soundtrack.
A common mix-up is confusing Lava with other Pixar or Disney shorts. Piper preceded Finding Dory, while For the Birds accompanied Monsters, Inc.; neither was the theatrical companion to Inside Out.