Ok, I just sent myself a .wav file, with the same mime type ("audio/x-wav").
If I save the attachment first, and open it, I get a choice of ES Media Player and Google Music.
If I don't save first, the file plays with Google Music (there is no app chooser).
This on a Galaxy Nexus with 4.2.2, both in my app, and in stock Android Email.
I can only conclude that ES Media Player is only able to accept files, not "content provider URIs" used for opening without saving first (again, both in my app and stock Android Email).