Other vocals
I've seen this movie many times over the years, and listened to Patsy Cline's recordings over and over. I just swear that's not really her singing Blue Christmas, or Bill Bailey. It's especially strange because in the case of Bill Bailey she did record it. I know that the earlier "performance" numbers in the movie were of later, smoother vocal recordings of hers, with new accompaniments produced by Owen Bradley.
But these two songs in the movie - the vocals sound like a talented imitator rather than the real thing. It's just a minor distraction.
"It's as if God created the Devil...and gave him...JAWS"