I think a lot of people have the experience where they're dreaming but they're frozen - unable to move to run away from something or towards something or whatever. This makes a lot of sense, since when you're in dream-stage sleep all your muscle movements are frozen, and just like a light or noise from outside of your brain can sneak into your dream (I once had a dream that a friend was excitedly telling me about the Red Sox home opener, which turned out to be my clock radio telling me the same information), the feeling of paralysis can sneak into your dream as well (I think! Any stray sleep scientists stumbling across this blog, please correct me as necessary!).
I had a weird variation on this experience the other night. I dreamt that I was on a subway with some (ill-defined) friends, and someone started singing the Hallelujah Chorus (sort of like one of those Hallelujah Chorus flashmobs you see on YouTube). I tried to join in on the soprano part (because no one else was singing it), but I could only make choking, squawking noises. I strongly suspect that if anyone had been in the room with me, they would have complained to me in the morning about how I had been waking them up with my weird chokey squawky noises. I think my sleep-paralyzed vocal chords weren't responding to my brain's message ("Sing 'King of Kings!', dammit!") and I worked that "my voice doesn't work" feeling into the dream. I can't think of another time when I've dreamed about singing; I wonder if this always happens?