Im actually writing up a post about sleep paralysis right now. I had it happen again last night, for the first time in years. It can be incredibly terrifying, but it can't hurt you. To shake myself out of it I always try moving something small, like my pinky, then moving the rest of my hand to try and sort of "push" myself up.

The croaking sound and feeling you described has happened to me, and I'm pretty sure it's just us snoring a little (it only happens on my back), because our bodies are still relaxed and asleep. The sound gets exaggerated as a result of the auditory and sensory hallucinations sleep paralysis can cause.. We're kind of standing on the edge of consciousness, so to speak.