Q: I've imported a midi file with 3 staffs and only the first staff plays.  How do I get all 3 staffs to play?

I Have a mid 2009 Mac with 8 mb ram running Yosemite.

Ive imported a midi file with 3 staffs.  Only the first staff plays.  I've tried muting the first staff and the play head line moves over all 3 staffs but  I hear nothing.

How do I get all three staffs to play?