Christopher Plummer was dubbed in the released film, but here's his own vocal track, with Charmian Carr.

From his autobiography, "In Spite of Myself":

"To further shake me up, our first recording session was upon us. Daunting is not a strong enough word to describe it. Julie and I stood side by side in a small glassed-in cubicle facing two microphones. Surrounding our prison cage sat 75 musicians like hungry jackals waiting to pounce, led by their keeper, Irwin Kostal.

Warbling softly into a mike is far more difficult than singing full out in a theatre as I was later to discover. One is much more likely to catch and collect "frogs" in the throat, whereas projecting usually gets rid of them. I tried so hard not to look like a complete basket case. Julie, sensing my nerves, took hold of my hand and held it throughout the session. It must have taken her days to recover the use of it afterwards, I had squeezed it so hard.

No matter how diligently I'd slugged away at my lessons, I was still untrained as a singer. To stay on a long-sustained note was, for me, akin to a drunk trying to walk the straight white line, whereas you can bet the very first cry that Julie let forth as she emerged from her mother's womb was in perfect pitch! Listening to the playback, there was no disputing we were on separate planets. In the end, Robert Wise managed to hire someone to take care of my elongated passages, and the balance was somewhat restored."

Bill Lee was the "someone" who was hired, and he replaced all of Plummer's singing, not just the long notes.