i haven't heard of that case, but it might have magnets that activate the "in holster" part of you ring profile. when the magnets come in contact with the copper-ish metal pieces on the back of your phone, this profile activates.
to fix this go to profiles ---> advanced ---> edit ---> phone...
the first part of the page should say "out of holster" and have your settings. then there is a separation and after it, it says "in holster" change those settings to the same ones listed above. i bet that part is set to silent.
hope this does the trick!