Sparkling Cyanide (1983)

A socialite while having dinner with her husband, her sister, her aunt, a friend of her sister and her husband, her husband's assistant and an English reporter, dies. It is later learned that she died from ingesting cyanide, Initially it is believed she may have given it to herself because she's having emotional problems. But her husband gets a note saying she was murdered so he tries to prove it and part of his plan to bring out the people who were there when his wife died. He gives a toast then everybody gets up to dance or do something. When they return, he takes a drink of champagne and drops dead. So they wonder how the cyanide got into his glass. Her sister who has taken a liking to the reporter learns he is actually an investigator. And he works with the police to find the killer.