Is piano-playing a subject in the book Midair?

No, piano-playing is not a theme in Frank Conroy's short story collection Midair. It isn't a theme in the title story either. There is a short story in the collection, "Roses," which focuses on an artist, but nothing about piano-playing.
However, music and the piano in particular were an important part of Conroy's life. In fact, Conroy was an excellent jazz pianist and performed in many jazz clubs. He even won a Grammy award in 1986.
Piano-playing is an important theme in another of Conroy's novels, Body and Soul. The novel follows a child prodigy gifted with incredible musical talent. This very same talent is both a blessing and a curse, as it ultimately leads him to discover that he has no space for anything in his life but music, not even love.