Do the rules state this explicitly - that a particular kind of looking at an illusion counts as interaction - or is this your personal interpretation?
Actually, you've got a point. Looking at something isn't really interacting with it. But moving through it -- or attacking through it -- is. Thus, you can't just magic up mist and have total concealment, 50% miss chance, automatically, though you could dissuade people from trying to attack you because it looks like you're shrouded in mist, so they think they'll have the miss chance. But attacking through it, or moving through it, almost definately counts as interaction (feeling the moisture, seeing the precise swirls when you move, watching the shadows, etc.), and so if they attack you anyway, they'd get a will save.
But is the creation of mud silent? I mean, aside from the noise of you casting the spell, spells themselves make noise when they go off. Fireballs go WOOSH, ligtning bolts go BOOM, and puddles of mud go GLORP. You can make something look like a patch of mud, but if you do it right in front of them, and it is silent, it's going to have a suspicious absence of GLORP (just adding Ghost Sound solves the problem, but you'd have to have it quickened to do it in the same round). Spells make noise when they come into effect, I think...even spells like Charm Person make a little tinkle of bells, I'd say, unless it was cast as a Silent Spell (which also means you don't have to speak).
If certain spells don't make noise when they go off, it seems like these are the exception, and should be pointed out, right?
I don't think I'm being harsh, I just think I'm not giving
Minor Image the ability to look like a spell being cast. Is that really so bad, when you can singlehandedly generate enough solid gold bricks to fill as many carts as you have on hand?
If you're working the illusion around yourself, for example, there's no reason you can't add the sounds personally -- who among us hasn't pantomimed projectile vomiting in some joke setting? "Blarrrgh!" Or have someone hiding behind the corner adding "ugh" and "argh" sounds every so often (just make sure there's plenty of distance between the actors and the audience).
That works, but I'd call for a Perform (acting) check or something. It's a far gry from going "Blarrrgh!" to sounding like your vomiting....speaking as an actor.