sorry to bug you so much but i guess receiving comments on pull requests will let you learn how to package using nix better.
you can actually reuse parameters inside attribute set so this expression would look like:
```
stdenv.mkDerivation rec {
  name = "gupnp-av-${version}";
  version = "0.12.4";
  src = fetchurl {
    url = mirror://gnome/sources/gupnp-av/0.12/${name}.tar.xz;
...
```
(notice the ```rec``` there)
then this is easier to upgrade next time since you only change version in one place.