Every touring program from the Marlboro Music Festival is formed around summer performances that co-directors Mitsuko Uchida and Jonathan Biss felt were rehearsed to a point of maturity and dynamism. Tonight’s program was created around Brahms’s first piano trio and Kate Soper’s dialectical piece for soprano and flute. Musicians at the beginning of their careers perform alongside experienced artists in a program that balances new music with the elegance of Schubert, Handel, and Brahms.