Joan Collins' singing voice(s)

Joan collins has two songs in this film. If I remember the first one correctly, she starts out alone and Bing walks out and joins her. It's a nice singing voice, but I don't think it's really her own. At the finish, she and "the boys" are singing "Teamwork" (or something like that), and Joan is doing a Rex Harrison/My Fair Lady/not-truly-singing performance, and I do mean Joan this time; it really sounds like her voice here. Anybody have any information about this inconsistency?

The GREEN HORNET Strikes Again!