Finding Dory is not out before 17 June (for the American audience at least), but you can listen to the theme song right now: Sia performed Unforgettable live it in the Ellen DeGeneres show. Watch it / listen to it below.
Finding Dory's Unforgettable is a cover version of a song originally performed by Nat King Cole in 1951, just like Finding Nemo's Beyond the Sea was a cover of a Charles Trenet / Jack Lawrence oldie.
The title and lyrics are a reference to Dory's forgetful nature; in the film, the blue tang (voiced by Ellen DeGeneres) remembers about her parents and sets out to find them, with the help of Marlin (Albert Brooks) and Nemo (Hayden Rolence is reprising the role of the young clownfish, who was voiced by Alexander Gould in 2003).
Finding Dory is written and directed by Andrew Stanton, with Angus MacLane serving as a co-director.