Walt Disney Records will release the soundtrack for Disney’s “Descendants” on Friday, July 31, the day the eagerly-anticipated movie premieres on Disney Channel (8:00 p.m. ET/PT).  The soundtrack pre-order is available now HERE.  It features the single “Believe,” performed and written by 16 year-old recording artist Shawn Mendes (Island Records) and original songs performed by the “Descendants” cast, plus bonus tracks and a score suite. On Tuesday, June 16, Shawn Mendes’ “Believe” single will be available as a song purchase…