Let me quote from Mark's blog almost one year ago:
March 5th, 2010 at 7:19 pm
Mark, will there be an update to the sound theme to match the updated visual brand?
Mark Shuttleworth: Gack, I completely forgot about that. A very good point. Would you see if you can rally a round of community submissions for a sound theme inspired by light?
Let's keep it short and sweet: What are the current considerations for the Ubuntu default sound theme?
Update, October 5 2011
This question was "answered" before the release of Natty. However, it proved to be wrong in the mean time. So the legacy continues, and I am reopening this one.
Update, February 15, 2012
Something is rolling! Looks like there will be a new sound for 12.04. Only a start-up sound however, no complete theme.
Update, April 30, 2012
It seemed to look good, but for some reason the sound did not make it into 12.04. There have been no announcements as to why, however. The sound theme remains a mistery...
Update, May 2, 2012
Mark Shuttleworth made a statement on IRC recently saying that it simply "was not perfect yet." It will maybe be in 12.10 then.
Update, December 6, 2012
Not in 12.10, either. But - there's still hope for 13.04!
As of the last update, the bug report for getting a new sound theme has been marked as Confirmed (but not In Progress) with the last comment posted October 31, 2011.
Update, September 11, 2013
Won't be in 13.04, and neither in 13.10. However, an interesting proposal has popped up. The composer of the "Mass Effect" soundtrack offered to make a startup sound for Ubuntu on this mailing list.
Discussion on Reddit here.