One Body Too Many (1944)

Taken from the IMDB : Insurance salesman Albert Tuttle arrives at the Cyrus J. Rutherford estate to sell the millionaire some life insurance. Rutherford is already dead and his heirs have gathered at the mansion to hear the reading of the will. Rutherford's will won't be read until he is properly entombed and the heirs are forced to stay on the premises or be denied their inheritance. Tuttle soon finds himself mixed up in shenanigans involving Rutherford's niece, secret passages, a missing body and murder.