Is your user a member of the audio group? When you login in runlevel 3, /etc/login.defs temporarily adds your user to the audio group, and that gets inherited by the X session when you run startx. In runlevel 4, login.defs does not apply and unless you are explicitly a member of the audio group you may lose access. The following command should work:
Code:
# usermod -a -G audio username
If you're not a member of the audio group then you are likely also missing some other important group permissions. The following command would add your user to the recommended groups:
Code:
# usermod -a -G audio,cdrom,floppy,plugdev,video,power,netdev,lp,scanner username