Alright so here's the rules question:

Making an adept with Killing Hands and Elemental Strike.

Killing hands says that I can take my attack and instead of causing Stun I will cause Physical Damage.

Elemental Strike lets me pick an elemental effect for my hands.

If I choose Smoke or Sound it does Stun damage as per Element rules, but Killing Hands takes my Stun and makes it Physical by choice.  So, would I be forced to do Stun as per the element or would I be able to still choose to do Physical damage and gain all the effects of the element as per Elemental Strike still?

The damage would be what the type of elemental strike calls for, as you are gaining all the nifty effects of said element.

I think the key language is "you may do normal Stun damage or declare the use of Killing Hands and inflict an equal amount of Physical damage instead."

This tells us that you have two options with killing hands.  You can either do "normal" stun damage, or do an equal amount of Physical damage.  What you are proposing is to choose between special (not normal) stun damage, or physical damage equal to the special damage.  Since that's not what the text lets you do, you can't do it per RAW.

I think that the text argument is also supported by game balance.  Smoke and sound are already super powerful because of how they negate armor.  Using killing hands to turn smoke into deadly toxic smoke, and sound into killer sound, seems undercosted at .5 power points.  In fact, technically, if that combination worked it would turn your fists into barrier and vehicle destroying monster weapons.  Somehow, your armor-ignoring smoke would cause walls to crumble and cars to explode.  It would make more sense to let sound shatter objects with ease, but I still don't think it would be balanced.  Barriers are literally helpless when their armor is ignored (they roll 0 dice to resist), and cars are all but helpless.  It would also negate the efficacy of the Blast damage type, as well as make Smashing Blow pointless.  Smoke and Sound are only balanced as is because vehicles and barriers ignore stun damage, so you can't automatically destroy them.