Previous owner filled in fireplace shelves with incorrect brick. Can I chisel them out?
You might not get the original look back, but you can make it better.  I'd take grinding wheel and cut a deep groove in the outer ring of mortar, then go to town on the bricks in the middle (as the groove will protect the older brick from damage).  This will make quite a bit of dust, a dust shroud and HEPA vacuum rental are highly recommended, along with closing off the room. 

After that operation, you may be stuck mortaring the inside of each nook smooth, as exposing the brick ends is a dicier proposition.  The new mortar, unfortunately, looks perfectly good and probably bonded quite well to the old brick.  The new brick and mortar may well be harder (even considerably harder) than the old.

Note the (now) traditional use for this space would for a flat screen TV.  

See the two small holes, one of them with a metal expansion bolt?  There may have been a heavy picture or mirror here in the past.