Even after you've completely downloaded the mp3 you still can't play it? Are you trying to play it through LW or from the desktop? Try dragging the mp3 onto SoundJam to force it to open the file. Or launch SJ and choose the Open command and select your mp3. I use iTunes and it works fine, I can just go to the Library tab in LimeWire and double-click on a song and iTunes automatically launches and starts playing. If you go into the Incomplete folder under the Library tab you can also play the songs that are currently downloading before they're even finished.
Oh yeah, here's 1.4 for you.
http://limewire.gnutelliums.com/1.4b/
As I said--it works better for me than 1.6 so I hope it solves your problems. Also, uninstall 1.6 before you install 1.4.
good luck