The only advantage to me personally in JB'ing it would be if I could get Amazon video streaming on it. Even with XBMC on a JB ATV2, I still have 720p with lossy audio. I need 1080p with HD audio to play my ripped BD's.
Does JB'ing it change the audio transcoding issue - will it stream bit-perfect audio via HDMI (legit question - I actually don't know)?
When it comes to the sound, I have no idea. I'm one of those people using the television's own speakers... I was just thinking about your comments regarding ATV2 not being able to access a NAS and saying a Mini with XBMC would be a better solution, and asked why you didn't just jailbreak it since a jailbroken ATV2 can run XBMC and access shared folders and disks in the network (like a NAS). But with what you're saying now I understand why you're not bothered doing it