Wait, why do we have to solve it again? Isn't the problem simply that the patch hasn't made it into Ubuntu packages, and the PPA wasn't built for precise?
First, a workaround. In your /etc/apt/sources.list, you probably have something like this:
http://ppa.launchpad.net/diwic/fluidsynth-test/ubuntu/dists/precise/main/source/Sources
I don't know the exact string; I'm guessing from your above error message. Note that it contains the word "precise". Ubuntu upgrader has a habit of changing all of your sources over to the new version name, which sometimes breaks if there isn't a version packaged for that release. So try changing "precise" back to "oneiric" and then re-running apt-get update. That might fix it temporarily.
For the long-term solution: what is the patch in question? Did dwic commit the patch into the Subversion repository, or did he just provide a test version in the PPA? If it's the former, can you identify the Subversion commit that fixed the problem? If not, perhaps it's time to put it into Subversion.
Of course, David can probably do a better job helping.