For those who have problems with stock music player accessing external storage you can install this one, its version 4.1.511 of Google's ICS stock player and it looks much better imo, plus, it has the ability to sync 20000 of your favourite songs via cloud. You can register at music.google.com (only for US). If you are from another country like me you have to use an US-proxy to register. After registration you can connect from everywhere, it works 100%
Beside that you can use it also without cloudservice, it plays music from internal storage, as well as from emmc mounted external storage
!! Before starting, make a backup of your music.apk !!
1. delete music.apk (maybe named com.android.music.apk or com.google.music.apk) in system/apps with Titanium or RootExplorer
2. copy com.google.android.music.apk to system/app
3. set permission to rw-r--r-- for this file (same as other apk in system/app)
5. goto settings-apps-all, select music and clear data.
6. start music and configure it with your google account.
7. check in settings-account-google if there is an entry music sync, then you're done
Nexus 4 - KK 4.4