If set the "Winamp Classic Interface" and close and reopen Audacious, it reverts back to the "GTK Interface". Is there a way to make the change to the "Winamp Classic Interface" permanent? I took a look in ~/.config/audacious/config and didn't see anything obvious. In ~/.config/audacious/log, I did see:
Selecting interface gtkui
Running interface gtkui@0x7ff393fb6860
Note: When I switch to the "Winamp Classic Interface", that doesn't seem to generate a log entry.
Fixed my problem: I noticed that several files in .config/audacious were owned by root, probably from when I was trying to get around the problems in 2.4 by running as root.