“I would like to ask about a speech and hearing impaired person using signing as a means of singing. Should they use their hands to express themselves when we sing? Or should they merely make melody in their hearts?”
Clearly, the command to sing is to the body of believers. “Speaking to one another in psalms and hymns and spiritual songs, singing and making melody with your heart to the Lord” (Ephesians 5:19). A person who uses signing as his or her primary language is “speaking” while they sign. Just as a person who speaks Spanish is “speaking” when he sings in his native tongue. The fact that this person is speech and hearing impaired in no way should limit his ability to speak in sign language.
I have been in worship services in which the deaf were singing by using their sign language. I was impressed both with their reverence and their desire to express their love for God by teaching in song, even though they were using a different language to do so. Making melody in the heart is for every person who sings, whatever their language. So, I believe that these good believers should sing out, using their sign language.