Can one perform a Domain Expansion without saying the invocation out loud?

Every time someone does a DE, they have to make a hand sign and state the name of the technique. Could it be done with only the hand sign?

For example, if a sorcerer is underwater and can't speak, or if they are mute, would they be unable to do a DE?

In S1, Megumi has trouble summoning his shikigami when he is concussed and can't form the proper hand signs. So I imagine that if a person is mute, they won't be able to do a DE, since the incantation appears to be a requirement.