Another place this is discussed is http://unixsadm.blogspot.com/2008/06/gstreamer-plugins-to-decode-mp3-files.html
The last comment tells you how to get the source so you can compile this yourself, avoiding having to go through the shopping cart/checkout business. (There is no up to date tarball at the link given; to get the up to date source, you need to use Subversion.) Another comment there suggests it is better to use Sun Studio than gcc to compile. If you have Sun Studio installed, you can do this by setting the environment variable CC to "cc" ("export CC=cc") and placing "/opt/SunStudioExpress/bin" at the front of your PATH before you run make. -- This message posted from opensolaris.org