You didn't say what *kind* of music you're after, but try FM 91.7, 97.9, 98.9, and 104.3. Some of these kind of mix it up between Latino and Brit/American music; 104.3 seems to be the most consistently Anglo.
Also, there's
http://www.pbcpanama.com/ and
http://www.coolpanama.com/