Translation:You are making music.
Playing an instrument.
To compose is "zenét szerezni". Literally, "to author music". Or "komponálni".
I am not sure that is correct. Writing music is composing but making it is more like playing an instrument, etc. But if you are a jazz musician, the two can definitely mix.
are the sz endings for "you" somehow related to the same thing in Slavic languages?
Slavic doesn't have sz for "you", though; they have s. (Using Hungarian spelling.)
On the other hand, it's tempting how m is pretty common for first person and d/t is pretty common for second person in languages from different language families.
Just to clarify: Does "making music" in Hungarian mean the same thing in English: to play instruments, to sing, or to do both? That is, to make music sound--by any means?