SPELLING DOESN'T COUNT

The four words below can be changed into new ones - phonetically - by adding the same two letters, in the same order.

SIN
MIST
LEER
TUNE

HINT:
The two letters are added at the end of each word. 

Answer:

The added letters are IC:

CYNIC
MYSTIC
LYRIC
TUNIC

Credit:
NY Times Crossword
Dana Delany and Matt Ginsberg
THAT'S DISGUSTING - Sept. 4, 2011