Does "'tis" means "this is" or "it is"?
"'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.
'My country, 'tis of thee.
A similar expression is 'tiwll
'tis is still used in English, even if it is chiefly poetic, or literary. It is the contraction of it is.