In the ballroom scene, you can hear a male singer that you never see; the same for the many musicians supposed to be in the place. Instead of all these artists, you can see a man alone with his piano. Maybe I missed something, or maybe the orchestra and the singer were hidden Under the long dresses of the dancers:)
Life is a long quiet river
I noticed that too. The ballroom isn't really very large so maybe that is why Ford banished the other musicians.
The smallness of the ballroom helped to emphasize Lincoln's awkwardness as a dancer. He takes over the center of the room while the other dancers circle the edge. He is able to fulfill his desire of dancing with Mary Todd "in the worst way". I thought it was a charming scene and shows us more about Lincoln's social skills than a hundred awkward conversations might have.