It's grayed-out right now because the s/w team has bigger fish to fry, mainly. Secondarily, it's also to avoid a lot of confusion that is likely when they do enable system sounds. Since many of us ONLY connect sound through the optical port and watch a great deal of DD content, we won't hear sounds anyway. Multiplexing system sound effects in real-time into a DD stream is pretty processor-intensive and it's something TiVo has never done either. So we're only likely to hear system sound during PCM audio, which is liable to confuse a lot of people.
As a slight aside, I was reading the blog of a developer associated with the MS HD-DVD add-on player for the Xbox 360 and he mentioned that multiplexing sound like that while displaying HD content off the discs uses all the hardware threads of the 3-core processor of the 360; this task it just about the most processor-intensive application out there for the 360, with the exception of one particular game (Gears of War).