Just went and read the Silent Image just to be sure before posting...and good GODS, "no larger than a 15' cube"?! Crimeny, you could make all KINDS of stuff with that. I was thinking waaaay too small.
1) The illusory terrain/cover (rocks, walls, bushes, etc...) is, obviously, the first thought and (as noted above) highly highly useful.
2) Surround your head...or, well, hell. 15' cube, cover your whole body if you want: the spell does stipulate you can make a figure "appear to walk"...for that matter, keeping everyone together, make the whole party appear as someone else. Masking the party as a group of orcs or hobgoblins marching right into the BBEG's stronghold is still going to make the same marching and weapons/armor clanging noises on their own.
3) Create doors/passages against walls to cause pursuers to slam into the wall. At 15' you could do two 5' archways 5' apart.
4) Slay guards at their post and create the image of them still being there, on alert, so as not to rouse suspicion from passerby's. No,
they can't speak/interact. But someone just passing by or looking from afar -checking the battlements, for example- to make sure guards are at their posts, won't know the difference. Meantime, the partry has taken out everyone on the battlements and moved on to the next phase of their mission while the mage/illusionist keeps the other guards/sergeants/onlookers fooled.
5) 15' cube...15' cube...ummmm...a small gatehouse, tower or cottage.
6) A good sized pond/fountain/lake/moat.
7) Copse of trees or hedgerow. Either as simple terrain or a mini-ye-olde-Massmorph for the party...unless you have a really huge -more than 9 medium creatures- party, fitting everyone within 15' shouldn't be that hard.
8) Make your cart and single pack mule appear to be a team of shining white stallions pulling a crystal and gold gilded carriage (again, as above, the sounds being made by the reality should cover the lack of sound from the illusion)
9) A huge hole, crevace or pit opens up before the mage/between the party and charging enemies. No one will fall in, obviously, but they should stop...even momentarily.
10) Faux "wall" spells or obscuring effects: walls of ice, fire (even without the crackling, most creatures should be given pause), fog/mist...heck a 15'x15' stone wall just bursts out of the ground, what are the pursuing hobgoblins going to do?
11) Make the glade containing the legendary Golden Oak that leads to the pixie kingdom (or a toadstool village of blue brownies ;P ) that the evil wizard is hunting for appear to be a simple empty glade/field (or simply a normal looking oak tree).
12) Make the mage/illusionist appear to float 10' into the air. One round of missile weapons and spells get directed at "that" guy (cancelling the illusion, yes. But misdirecting attacks for a round).
13) Creating 15' area section of halflings (just standing there) to distract/move off the bullette that just burst through the ground.
14) Even without the sound, for...less intelligent or innately cowardly creatures, it's worth a shot (this, obviously depends on how well you know/work with your DM and vice versa) to try to scare creatures off: creating a huge owlbear in front of a bunch kobolds, a horrific looking demon/monster in front of one or a small number of goblins....any creature that the party has discovered some other denizen of the dungeon is afraid of/wants to avoid.
15) Maybe something of a DM's call...but no different, really that creating other terrain...can you create an area of shadows/darkness for PCs to hide/use stealth in?
16) Balls of colored flame or light...appearing, perhaps, to bob and weave and act, generally, like a will-o-wisp or mundane light sources (torches, lanterns, etc...) to lead creatures away. Essentially, can duplicate a Dancing Lights spell.
17) Making a treasure [15' cube] room appear empty...or making an empty room appear full of treasure. Or trick others into believing a particular treasure is missing or still there after it's taken.
18) Turn a drab tavern room or any 15' cube area into different surroundings...as to illustrate a bard's performance/tell a story/put the audience "inside" the story.
19) Make a character/creature appear to change into a werewolf (or other lycanthrope), basically #2 + #14 with the base character moving around and making menacing sounds ("Raaar. Grrr.") while their appearance seems to change. Worth a shot in a tight situation, one never knows...do one?
20) Hell, 15' cube, a dragon flying in the air! Just have them soaring around, no wing flapping necessary (though I would argue the spell does permit for it).
Basically, anything you can possibly imagine that will fool and/or distract within a 15' cube, 60' range for [up to] 10 minutes.
"Limited" by the lack of sound? Easier to overcome/see through..mayhaps...but to a true adept of trickery and illusion, hardly a spell of limited potential.