Robert Preston never did any singing before The Music Man?
According to several biographies I've read online, not only had Preston never played a musical role before, but he had never sung in public before. For someone who did not have an extensive music background, I think he more than delivered. His rhythm and cadence were remarkable. "Ya Got Trouble" is very difficult because it follows a frequently changing meter that allows for very few breaths. "'Til There Was You" required a classic, romantic timbre and he had to be able to hold his own against Shirley Jones, no easy feat.
As far as I'm concerned, he sounded wonderful. Though I like Matthew Broderick, I find it crazy that Broderick has the training and experience as a long-time musical theater singer and yet he couldn't hold a candle to Preston. Preston may have lacked vocal experience, but he more than made up for it in raw talent, proper characterization, and a desire to train and polish.