From Mordenkainen's Tome of Foes comes the Balhannoth (p. 119). In the Languages part of its statblock, it says:
Languages understands Deep Speech, telepathy 1 mile
Normally when a creature speaks a language it would say just "Deep Speech". When it can understand it but cannot speak it, it would say "Understands Deep Speech, but can't speak" or "Understands Deep Speech, but doesn't speak" (which, effectively is kind of the same thing I'm assuming).
Now in this case it only says "understands". I have a gut feeling that this creature can't speak. Can anyone here confirm this somehow?