With a bit more creativity, you *can* work around the sound limitation for silent images. ... 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).
And let's not forget the possibility of the illusionist standing directly behind the illusionary creature itself -- sure, swords and arrows are going to slice right through the phantasm, but that still implies that the mage wasn't the primary target and so he's going to be tougher to hit for it.
Idea for Magic Mouth: prep it at the entry point where you expect the enemy might pass on its way to attack you. Set the trigger for something simple, like an attack on your party, or five seconds after the creatures pass by. Sound? "Psst, hey, bub," or "Run away!" or anything to distract the bad guys for a moment.