I'm sorry, but as a 40 something male myself, I can't imagine anything more squirmworthy and I have to question why exactly you want to pour energy into such an unlikely endeavour. There are choirs around - if you want to go to one, then do it YOLO - but why try to inflict it on the office? I do sing or might sing: At karaoke, at a football or rugby match, to the radio, round a camp fire, half drunkenly down the pub, to accompany my half competent strums on the guitar - but joining a choir? with implications about performance and arranging get togethers to practice, with the guys I work with? You are having a laugh.
Invite them all to the local pub's karaoke - along with their wives and girlfriends. Take it from there - but even then be aware that a one off karaoke may get some attendance, but that ain't necessarily meaning that they want a choir. And if there is a lack of interest in pub karaoke, over and beyond the usual level of interest in meting up for drinks down the pub, then I think that gives you your answer. Good luck.