Soundtrack piece at the grand ballroom scene

If you mean the music at the end of Episode 1 when Prince Andrei asks Natasha to dance, it is the Waltz from Khachaturian's music for Lemontov's play Masquerade. The period is a little off for War & Peace, but the choice of music is somewhat appropriate as it premiered on June 21, 1941; Hitler invaded the Soviet Union just a few hours later. Lermontov's play is actually much older (1835).

There is a Chopin-like piece much early that the majestic "Masquerade Waltz" and they didn't credit that either. Frustrating.