Final answer:
The musical that Don's best friend suggests turning the Dueling Cavalier into is called Singin' in the Rain.
Step-by-step explanation:
The musical that Don's best friend, Cosmo Brown, suggests turning the Dueling Cavalier into is called Singin' in the Rain. In this musical, Lina's voice is overdubbed by Kathy, leading to comedic situations and a successful transition from silent films to talkies.