It might be better to apt-get source libsdl1.2debian after you change to the feisty repos. This will fetch the sources and will allow you to change back to edgy, so you can backport it yourself.
In fact, I just did that, and it works. Here's what I did:
In terminal: sudo apt-get build-dep libsdl1.2debian
Then I changed the repo location to feisty, as you said.
In terminal: sudo apt-get update && sudo apt-get source libsdl1.2debian
In terminal: cd libsdl1.2-1.2.11/
In terminal: sudo fakeroot debian/rules binary
After the build was done, I changed the repo back to edgy. I then installed(with dpkg -i) the built package of my choice(libsdl1.2debian-alsa where you downloaded the sources.
To fix unmet dependencies, install the built libsdl1.2-dev and libsdl1.2debian(base package) along with your libsdl1.2debian-soundprovider package. You should have no problems with dependencies with the edgy repo later on.