Dont know for sure, sourceforge probably for a source code tarball if you wanna compile it, you will need the toolset to compile stuff from source though cause Suse doesnt come with that either. Best bet is probably an rpm depository somewhere, there are quite a few Im sure. Better take a close look at what exactly is on your system and what your missing first before you start blindly installing stuff.
You should be able to get away with using YaSt I think if you can figure out what its lacking. Might wanna try searching about this at linuxquestions.org, im not a Suse person, I dont like distro's that are rpm based or lacking lots of things right from the start. They work just fine, just take some work to get them to do everything the way I like.
Should be able to try to play an mp3 from command line with the alsaplayer, just type play <filename> or alsaplayer <filename> and see what happens, if it spits out an error about missing a library or unknown file format or something like that, then have a look in what is installed on your system and then search for whats missing.
One of the benefits of command line apps is they spit out actual errors in text format when you run them, makes debugging easier. Most GUI apps can be started from command line as well, they will usually spit out debugging info too. Its a useful trick for stuff like this.