This is spoilery as well.
Was there sound in space? I honestly can't recall. I was concentrating really hard on being tense and afeared whenever they were going through Reaverspace. I do, however, think that it was an excellent decision NOT to show whatever was on the vidscreens when all that screaming was going on. Whatever the Reavers do, whatever they broadcast, we had to read if off everyone's faces. So...I can't remember, when Mal blasted the creepybugship with the hastily mounted cannon, was there a noise? I can buy that there might be able to be sound when you had the Reavers facing off against the Alliance, that the ion cloud might have collected enough gas to effectively conduct soundwaves.
I just wish I could remember. It would tell me whether or not Joss had compromised on this.