Can braces make a lisp go away?

It will depend: Air that escapes in the form of a lisp may be corrected by braces if the front teeth can be brought together by the orthodontics. However, it may not correct the lisp is the functional bite position is such that the front teeth are not close enough for approximation. Check with you doctor to determine your best functional bite position and then determine how close the front teeth come.

Typically no: Speech is a "learned behavior" and although structural change (such as with braces) may effect some improvement, it should not be a reason to seek orthodontic treatment.