No matter how you cut it, it's an awkward moment in the series. If it's just a stupid mistake, it's hard to believe that something that basic would go uncaught by anyone involved, on the other hand, if it was intentional, more explanation was certainly called for.

However, I'm willing to cut Whedon a little slack for being a pioneer (as far as TV) in portraying the vacuum of space as silent, no matter what's crashing, blowing up, or "swooshing" past the camera. I have to assume that every other series/movie knew better, but just couldn't resist putting sound effects in anyway, assuming we were all to stupid to catch it.