Analysis of voice quality of film

Scenario: Ek, Friesen, and Scherer ( 1970) tested whether lying influenced one’s voice quality. Participants were randomly assigned to view either a pleasant film or an unpleasant film, but all of the participants were asked to describe the film they saw a being pleasant. ( thus, the  subjects who  watched  the unpleasant  film  has  to lie  about  what they saw )

An analysis of voice quality showed that participants used significantly highly voices when they were describing the unpleasant film rather than the pleasant film. Why cant the authors conclude that lying produced differences in voice quality?

determine the possible confounding variable(s) (there may be more than one), and consider how they might be eliminated using research designs presented in the readings (e.g., 2×2 factorial design).