Mathematica usually uses LeafCount to decide which of two alternatives is "simpler", smaller count means simpler.
So Mathematica thinks your preferred form is more complicated. That means something more than Simplify will be needed.
Here is one method you might use
Out= Sqrt[a*b]*Sin[c + x]
This even works when you do not literally have a,b,x, but the same variable or constant must appear on both your x positions.
Out= Sqrt*Sin[c + q]
You might want to enhance this by using the appropriate expression for c as a function of a and b.