This article is from the Dreaming FAQ, by "Lars Rune Foleide" firstname.lastname@example.org .
Your whole body is normally paralyzed during REM. Your senses remain
active, and sometimes it happens that you partly act out your
dream. So you may talk while sleeping if your muscles aren't fully
paralyzed. We know that we can dream in non-REM sleep, maybe not as
vivid, but enough to talk. Most sleep talking occurs in non-REM. In a
Lucid Dream can you also command yourself to talk while dreaming.