Umm...actually I didn't know what app I should choose to clear default...
You see, I was trying to change my phone ringtone & notification sound, usually I got 3 options from where I want to pick the sound, but then I accidentally click "always" instead of "just once"..
so in all apps,which one should I choose?
There are just sound enhancement & sound picker, but both of them are "no defaults set"
Thanks b4