Installing app from google play in my SD Card
Have you tried the "Apollo Music Player"-app by the CyanogenMod team? As far as I know you cannot read and play music from the external memory if the application is not able to. So you won't fix it by installing the app on the external SDcard.

The Appolo Music Player is:


Free 
Great
Equalizer
High Quality


This cannot be downloaded from the Google Market, but you might grab it from XDA (just be sure to enable to permit installing from unknown sources):

http://forum.xda-developers.com/showthread.php?t=2196385