“No decoder found for (filename)”
There is no plugin installed for the type of file you are trying to play. If you installed Audacious through a package manager, you may need to install additional plugin packages and/or optional dependencies. If you compiled Audacious from source, some plugins may not have been compiled because of missing library dependencies. Run ./configure for audacious-plugins and look for any lines in the output that read “checking for (library name)… no.” Install those libraries, run ./configure again, and then recompile audacious-plugins.
The GTK interface is ugly and doesn’t fit it with my desktop theme
Window positions are wrong in the Winamp Classic interface
(That is, after closing and restarting Audacious, or after minimizing and restoring, some of the player windows have moved.) This is a bug in your window manager. Ask the developers to fix it or use a different window manager.