Does it matter whether I say my prayers out loud, or can I just say them in my heart?

The Lord has said we should do both: “I command thee that thou shalt pray vocally as well as in thy heart” (D&C 19:28). In the Book of Mormon, Amulek taught that we should “cry unto [God]” in many circumstances in our lives and that “when you do not cry unto the Lord, let your hearts be full, drawn out in prayer unto him continually” (see Alma 34:17–27). There is power in praying aloud, so although silent prayers may be more comfortable for you, you should try to give voice to them when you can.