I'm going to go with my gut and suppose that William Demarest was only pretending to play the cello or bass fiddle (I'll leave that to our resident music experts). Was the music dubbed or did Demarest really play the strings?

Here he is with the cello:

https://www.pinterest.com/pin/william-demarest-a-journey-through-vaudeville-broadway-films-television--214765475984101095/