Air carrier Captain reported executing a missed approach due to an aircraft on runway.

Narrative

After switching to Tower during our approach; we could hear Tower trying to clear an aircraft onto our landing runway for departure. The controller said he had tried to contact the aircraft with no response. We could hear the aircraft respond to this transmission; but it was weak and Tower couldn't hear it. There was confusion and the aircraft started its takeoff roll too late for us to land. Tower cancelled the takeoff clearance and told us to go around. The FO (First Officer) executed a textbook go-around and we returned for landing.The radio from the departing aircraft was very weak; causing confusion from the Tower. The only transmissions that Tower could hear were from a much louder secondary radio perhaps on guard or just a different pilot with a louder voice. Everything was done correctly; except that I should have notified the Tower and the other aircraft that the issue was caused by a weak radio.