That's a good observation and one I've wondered about also. The tonal quality or harmonic content of the voice depends on many factors, one of which is the condition of the vocal cords. That's why many singers drink lots of water, to keep them well hydrated and elastic.
However, if they become dried or inflamed (as in laryngitis or overuse), their effective length, elasticity, smoothness and thickness will change, changing the quality of the voice. So it may be that the smoke filled air you breathe causes your cords to become mildly inflamed, changing the quality of your voice, which you find pleasing.
I myself notice that sometimes in the morning if I sing shortly after I wake up, that my voice has a more gritty and rough quality which I too find pleasing, probably due to some mucus and boogers around the cords which have accumulated overnight. Unfortunately it soon disappears. I've thought of spraying some slightly inflammatory substance, such as vinegar, over them to see what happens, but haven't had the guts to try it yet. I'd hate to have them swell shut.
In your case, I don't know what non-toxic substance you could substitute for the smoke. Just play in bars until you find out.
