Well, I too have problems picturing Clark singing. Yes, I remember the scene in "My Best Friend's Wedding", but somehow I just can't picture Clark doing it, whether or not he can sing a note. I don't even remember - has it actually beome established in fanfics that Clark can't carry a tune? I know that Dean can't sing, which is probably why I have problems picturing Clark singing, but I don't remember the issue coming up in fics that often. Maybe it's time to reread some more...
At any rate, I'll be curious to read the story, and I agree with my namesake - the esteemed KathyB - that old standards would suit them well. For Clark, how about
The Way You Look Tonight . And for Lois -
Fly Me to the Moon 
Kathy
