Hey Mad, nice if you and Rick can work it out, he does have a lot of knowledge to share.
I'll share a little as well, Rick can go into more depth but here it is in nutshell:
soundproofing = isolation
high frequency isolation = airtightness
low frequency isolation = mass + airtightness
If your intent is to sing in the wardrobe anytime you feel like it and not have the neighbors get excited, I would drop it. I doubt you can economically soundproof the wardrobe, and if you can, I doubt it will sound good inside because it will be too small, cramped & hot.
Sing when they are gone, or obtain the means to record portably and sing somewhere else.
Another note: when you use slang and do not bother to capitalize or punctuate appropriately people will not take you as seriously as you might wish.
cheers,
C.