Methods to trigger a gallery/music player refresh?
To refresh the list of your music you do following:

Go into settings->manage apps->ALL services. Scroll down and select "Media Storage." Then hit the "Clear Data" icon.

Source: http://www.droidforums.net/forum/droid-4-general-discussion/220965-players-dont-detect-music-my-sd-card-after-ics-update.html