"Listen! Dad is talking in his sleep."
Translation:Lyssna! Pappa pratar i sömnen.
Added that, it's an acceptable version. But actually both previous posters are mistaken. pratar i sömnen is much more common than talar i sömnen. A corpus search gives 16 hits for talar vs 227 for pratar.
talar is more formal so that expression may be a better fit for medical contexts. In everyday life, pratar is more idiomatic.
While I'm at it, cc08_ is right that i sin sömn sounds too strange. prata i sömnen is a fixed expression.