What about wav files from the original consoles?
How about the sound of blowing on an NES cart and struggling to get it to work?
Hrm... something like this is so subjective to individual tastes and thematic goals (layout, background music, etc.) but I guess I see what you're shooting for. I would imagine anything (movies, music, tv) that is even remotely game-related would be prime fodder. Going back to the NES, someone should do a "Wizard" layout...
As for my cab, I went custom; rather than put myself in it, I called in a favor to one of my classmates from grad school and she did some great voice work for me. My cab has an "announcer" I guess (think like NBA Jam, or NFL Blitz) and she's a hottie. Thematically, we settled on a younger, sexified version of the British lady from The Weakest Link, so my cab also has a bit of a sadistic streak. Lets just say quitting a game does not please her!
Knowing that the plugin is going to come out sooner or later, I had her do a bunch of different takes of a bunch of appropriate (and inappropriate!) phrases. When it does finally come out (looking at you, loadman!
) all I have to do is point it at the folders, and I've got instant variety.
I know I'm kinda bragging on it right now, but a cohesive theme REALLY ties together a layout, no matter what you use.