[1] The episode begins with the gang celebrating Monica and Chandler's engagement, after Ross finally shows up after missing the proposal.
He is auditioning for the role of a 19-year-old, which is 12 years younger than he is, and his efforts to "de-age" by dressing and acting like a teenager are met with ridicule.
Phoebe, on the other hand, would love to sing at the wedding, and badgers Monica until she agrees just to get her to get dressed for the night.
The two ex-lovers protest mightily, and Monica is almost convinced when Phoebe comes in, immediately misinterprets the situation and asks if she can sing at Ross and Rachel's wedding too.
During the tag scene, Phoebe sings in the apartment when Chandler comes out of the bedroom having finally had sex with Monica.