I got the same errors...
This is how i solved the issue->
1. Get the latest 2.4.0 sources (player+plug-ins) from -> http://audacious-media-player.org/
2. Get the slackbuilds from the Slackware DVD (player
3. Change the Version number.
4. Run and Compile.
5. upgradepkg --install-new *.t?z
6. ENJOY Audacious 2.4.0!
Edit 1. Compile and install the player first and then the plug-ins.