Sextet Number 1
From Memory Alpha, the free Star Trek reference.
The Sextet # 1 in B-flat Major is a work of classical music composed by Johannes Brahms in the late 19th century.
The second movement (Andante, ma moderato) was performed during a Mozart recital in honor of Vulcan ambassador Sarek aboard the USS Enterprise-D in late 2366. The music brought tears to Ambassador Sarek's eyes. (TNG: "Sarek")
