My HT seating surfaces are all soft leather. The only sound it affects is that of the person sitting in it...."Ahhhh!"

Sound is affected by all surfaces, but mainly the perpendicular and parallel ones to your speakers, not the ones you sit in. Be conscious of the first reflections, the bass traps in the corners and ceiling/wall junctions, the glass coffee table in front of you, the bookcases behind you, etc....way more than leather vs cloth...IMO.