prevent harmless but annoying 404 errors for sound files when sound is off

See lefnire's comment https://github.com/HabitRPG/habitrpg/issues/3903#issuecomment-52991242
After running the initial migration `db.users.update({$set:{'preferences.sound':'off'}},{multi:1})`, shouldn't be any users with `!user.preferences.sound`. But hey, better safe than sorry! Thanks for the fix!