"The Kid" is the talented but troubled frontman of his Minneapolis-based band The Revolution. To escape his difficult home life – his father is verbally and physically abusive, and his mother is emotionally abusive – he spends his days rehearsing and his nights performing at the First Avenue nightclub. First Avenue's three house band slots are held by The Revolution, the flashy Morris Day and his group The Time, and Dez Dickerson and his group The Modernaires. Morris, aware that The Revolution's guitarist Wendy and keyboardist Lisa are frustrated by the Kid's unwillingness to play their compositions, lobbies Billy Sparks, the nightclub's owner, to replace the Revolution with a girl group which Morris is already forming. He targets the Kid's girlfriend Apollonia – an aspiring singer and new arrival in Minneapolis – to lead his group, and tries to persuade her that the Kid won't help her because he's too focused on himself. She eventually joins Morris's group, which Morris names Apollonia 6. When she reveals her partnership to the Kid, he becomes furious and slaps her, as his father had struck him earlier.
At the club, the Kid responds to the internal band strife, the pressure to draw more crowds, and his strained private life with the uncomfortably personal "Darling Nikki". His performance publicly humiliates Apollonia, who runs off in tears, and angers both Morris and Billy, worsening his situation. Billy confronts the Kid, castigating him for bringing his personal life onto the stage and warning him that he's wasting his musical talent like his father did. The debut of Apollonia 6 is a success, and Billy warns the Kid that his First Avenue slot is at risk. The Kid seizes Apollonia from a drunken Morris and the two argue; Apollonia then abandons him. Returning home, he finds the house in tatters, with his mother nowhere to be found. When he turns on the basement light, his father – who had been lurking in the basement with a loaded handgun – shoots himself in the head. In a frenzy after a night of torment, the Kid tears apart the basement to release his anger, only to find a large box of his father's musical compositions. The next morning, the Kid picks up a cassette tape of one of Wendy and Lisa's compositions, a rhythm track named "Slow Groove", and begins to compose.
That night at First Avenue, all is quiet in the Revolution's dressing room until Morris stops by to taunt the Kid about his family life. Once on stage, the Kid announces that he will be playing "a song the girls in the band wrote", dedicated to his father – revealed to be "Purple Rain". As the emotional song ends, the Kid rushes from the stage and out the back door of the club, intending to ride away on his motorcycle. However, before he can mount his motorcycle, he realizes that the crowd is thrilled by his new song. The Kid returns to the club, to be greeted by the approval of his fellow musicians and the embrace of a teary-eyed Apollonia. The Kid returns to the stage for two encores with the Revolution, to the wild approval of the crowd (even Morris); overlaid scenes show the Kid visiting his father and mother in the hospital and sorting his father's compositions in the basement, accompanied by Apollonia. A montage of all the songs plays as the credits roll.

Question: What did The Kid find in a large box in the basement?
Answer: his father's musical compositions