Buble recently confirmed the news, saying, "By all accounts it would be true. I just really am a big fan and so myself and my camp, we wanted to do this with her and we were thrilled that she was excited about it too."
Witherspoon took voice lessons to prepare for her role as country singer June Carter in 2005's "Walk the Line", reported Us magazine.
The mother of three - Ava, 13, Deacon, 9, and Tennessee, 2 months - had then said that recording the album for the film was "the most challenging, horrifying experience of my life."
Buble's sixth studio album is expected to be released in 2013.