System boot chime right speaker only
This seems to be a problem for a particular number of MacBook Pro models starting in 2010.  According to this article in the Apple support site, the following models have a sub-woofer speaker in addition to the left and right speakers to enhance mid-range sounds. 


MacBook Pro (15-inch, Early 2011)
MacBook Pro (15-inch, Mid 2010)
MacBook Pro (15-inch, Late 2011)
MacBook Pro (15-inch, Mid 2012).


Looking at this article in the Apple discussion boards and this article here on Ask Different, users of these models are experiencing the same issue. If I had to guess, every model with this sub-woofer acts the same way and only a few users have noticed it. 

There may not be a way to correct this. I hope this information helps.