Why can I still hear signal from my mic when the gain is set to minimum?

Do not worry - this is normal behaviour. Note that for mic inputs the input gain range is +13 to +60 dB. This means that when the knob is fully left the signal still has 13dB of gain applied to it. The pre amp gain pots are not like faders on a mixing desk which will cut the signal completely at the minimum setting - to cut the signal you will have to mute the input on the next device in the chain or disconnect the mic.