There are some applications like Soundex that will do what you want at a
higher level. If you give Soundex Smith it will find smyth, smythe, etc.
However it does not work  at the letter by letter level and is rally a search
process rather than a comparison.

I do not know of anything which will do what you want - but there may be
something out there somewhere.

Another thing you would need to allow for is letters  out of order. Simth for
example.