i dont think ur ipad has got problem lol

i've this problem on my ipad 1 & 2 as well i did make some experiment, i on music, it fails to mute, i on asphalt 6 it fails to mute, i on PvZ it works lol. so i think it is probably a bug thats y i got the button to use it for screen rotation lock. to off sound i would just use the volume button