Samuel French will own them, if MTI doesn't. and if SF doesnt, the R&H Library could.
and if the R&H Library doesn't, then Dramatists Play Service Inc. could (known for owning the rights for much smaller lesser-known musicals)... and if they don't, then it would probably be the author's estate/the authors themselves
There are a lot of other licensing companies aside from MTI as FIERCEY pointed out. If MTI doesn't have what you're looking for then one of the others may have it. It would help if we knew what show you're looking for. Then we could just tell you who has it.