Our ablility to locate the source of a sound in three dimensional space is known as binaural localization and is based on subtle differences in intensity, time delays, and frequency colouration of the sound reaching each of our two ears. 
Just as visual depth perception can be impaired to two dimensions by restricting sight to one eye, you can experience the impairment of binaural localization cues by blocking one of your ears and paying attention to the resulting perception of sound.  In most cases it will be much more difficult to locate the spatial sources of sounds reaching one ear than with two.