If your girl doesnt moan, maybe she doesnt like to moan. Im a person who is not inclined to do it or much like it. I think moaning is its own thing. Apparently you wish you could make your girl moan. Apparently you would like to hear some moaning. I suppose you might bring up with your girl how it is that you cant help but hear these girls moaning and its a turn on so if she would like to moan it would please you.
I dont think this discussion would be too odd.
However, your behavior in reaction to your housemate is a bit odd; dont get me wrong, Im not meaning to be judgmental, I know a thing or two about being odd myself. More importantly, I dont think this is working to your benefit. This business of listening at the door coupled with your clearly odd behavior of checking his used condoms has you expressing emotional response to him and his imagined maleness independently of his girls.
Whether your housemate has a big cock or not, may have little to do with the moaning of his girls, but even if it does, so be it. If he doesnt have an 8 dick, some guy does. Reading between the lines I think you are telling us you are worried about your size in a way that I think is not good for you and your welfare.
5.5 inches is quite normal if not large. Many, many, women respond to us men because of our masculinity as we express it in the ways we act the way we might strut, or talk, or do things. Lots of girls like that about us. Here at LPSG we meet women who are specifically attracted to large guys, but many women do not respond that way.
The equipment you have is what you got. Do you have any reason at all to think your girl wishes you had a bigger tool? In the long run, I do believe a guy with average size equipment probably should get himself a woman who responds to male attributes other than big size. But dont think it should be a major problem. As a gay man, Ive responded to tons of hot guys who are irresistible sex machines with very average size dicks.
All things considered I think you should try to separate yourself from these thoughts and worries. If you cant get it off your mind, consider moving elsewhere.