Which preposition to use with alto

in Occurrences 4%

Her deep voice came out so strongly that she was able to carry the alto in the singing against the whole camp; she improvised delightful harmonies that put a thrill into the commonest tune.

by Occurrences 1%

In learning new music, voices should be rehearsed separately; that is, all sopranos, tenors, basses, and altos by themselves first, then combine the voices.

Which preposition to use with  alto