"'Tis" is a contraction of "it is", from all of the dictionaries I've searched. The little apostrophe just before 't' shows that there's a missing letter(the way can't shows that there the missing letters 'no'). Thus, "'tis" in your example would it : It is the voice of the lobster. Other examples are : 'Tis the season to be jolly. More @Wikipedia
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