Once you've started MediaPlayer and successfully navigated to your microSD card, you can load your registry and modify this key to use the microSD card by default.

Go to HKEY_CURRENT_USER\Software\Microsoft\MediaPlayer\Preferences\ and set "LastLibraryIndex" to "1" instead of "0".

This hack only works if you first tell MediaPlayer to play a sound file from the phone's memory and *then* from the microSD card. Restart your phone, and reconnect with CeRegEditor, and you'll see the flag to alter.