Audacious Developers have decided to switch back from GTK3 to GTK2 due to difficulty to write ‘traditional’ PC / desktop program using the toolkit.
John Lindgren posted in the audacious forum.
As anyone using Git master has noticed (or will notice soon), the GTK+ interfaces are now using GTK2 again rather than GTK3. The change was made because certain design choices  in recent releases of GTK3 have made it increasingly difficult to write a “traditional” PC / desktop program using the toolkit. The long-term goal is still to switch to Qt; however, the GTK+ interfaces need to remain stable in the meantime, and going back to GTK2 appears to be the only way to achieve this.
The 3.5.x branches of audacious and audacious-plugins are still using GTK3. There are also new gtk3 branches which will be kept up-to-date with the master branch. If there is enough interest, we might provide separate GTK3 tarballs for the 3.6 release. Let us know if you are interested in continuing GTK3 releases.
 A few examples of non-traditional design choices: monochrome icons (or none at all), client-side window decorations, and message windows mimicking Android.
Image Credit: Audacious Developers