We call a piece of music with voice a song. What do we call a piece of music without voice? We sing a song but play a XXX.
The term is "instrumental".
(music) Pertaining to, made by, or prepared for, an instrument, especially a musical instrument; as, instrumental music, distinguished from vocal music.
An instrumental is a musical composition or recording without lyrics or singing, although it might include some non-articulate vocal input; the music is primarily or exclusively produced by musical instruments.
However, it doesn't fit well grammatically in your example sentence, which is understandable. One "sings a song" but one would not say he or she "plays an instrumental", unless it was somehow important that you distinguish the fact that you're excluding voice.
Musicians would more likely not make the distinction that the piece has vocals or not, and say:
- I am playing a song. (informal use of the word "song" includes any musical piece)
- I am playing a piece. (mostly used for classical)
- I am playing a tune. (mostly used for jazz and pop)
- I am playing a number. (mostly used for jazz)
In general terms, you can call it instrumental music, but it would be more helpful to describe it in more detail, using terms such as symphony, chamber music, concerto and so on.