This was already explained in the show. Clark has to focus his superhearing and vision in order to use those powers. Otherwise, he goes about life just like everybody else. He had to teach Kara how to use her superhearing. If superhearing was a power that was "on" all the time, she would have been in pain the moment she exited her spaceship. This was made VERY obvious when she had to focus in order to use superhearing, and then further concentrate in order to hear something specific. If Clark and Kara heard *everything* ALL of the time and couldn't shut it off, they'd go mad.
There also appears to be an instinctual component with Clark, because he sometimes hears people calling for help even when he's not concentrating. This last happened in Bizarro when he heard the call, had to concentrate in order to isolate what it was and where it was coming from, and then realized it was Chloe!