Power word stun and power word kill, having only a verbal component, are fascinating, in that the intent appears to be that their power is tied to the words in question. What happens if you take out those words, through the use of the Silent Spell metamagic feat?
[RPG] Can a Power Word spell be cast silently
dnd-3.5espells
Related Solutions
Core D&D books have always deliberately stayed silent on the issue. (Get it? Silent? I crack myself up.) In Pathfinder, for example, the rules just say
Verbal (V)
A verbal component is a spoken incantation. To provide a verbal component, you must be able to speak in a strong voice. A silence spell or a gag spoils the incantation (and thus the spell). a spellcaster who has been deafened has a 20% chance of spoiling any spell with a verbal component that he tries to cast.
It doesn't specify language, format, length, etc.
Probably the most famous example of D&D verbal components, however, is the mage Raistlin from the Dragonlance novels, who always cast Light with the word Shirak. Go here to see a relevant quote from The Soulforge. There's some debate over whether that's a spell cast or a spell activation command word for his staff, but he also casts Sleep on someone with the words "Ast tasarak sinularan kyrnawi." That's their approach; most other gaming fiction BSes it away as "arcane syllables roll off the wizard's tongue..."
In general D&D tries to leave this undefined so that if in our campaign world you want arcane words that's fine, or if you want incantations or whatnot like the anime Slayers you can do that ("Darkness beyond twilight. Crimson beyond blood that flows..."), or Ars Magica real/foreign spell words ("Perdo Ignem!").
So the real answer to this is 'decide how you want it to work in your campaign!'
Having said that, a more recent example of "what spellcasting works like" is available in the Pathfinder comics (overseen by Paizo and some written by Paizo employees). There they appear to have made the artistic decision to use several of these models, the "weird words" model for arcane spellcasters and the "Slayers" model for clerics. See these panels starring the Pathfinder iconic NPCs:
You might not even need Subtle Metamagic
The Knock spell states that:
[...] When you cast the spell, a loud knock, audible from as far away as 300 feet, emanates from the target object [...]
As long as you are outside the Silence when you cast the spell, but the object itself is inside, the Silence spell will absorb the loud noise.
Knock has a 60ft range, so it should be perfectly doable to stand outside the Silence field and cast the spell into it.
Best Answer
There is nothing special about these spells that prevents you from applying the Silent Spell feat, by casting them with a higher spell slot, or the equivalent of such. They still require concentration and the appropriate action to cast, however.