Audacious - OLD, PLEASE USE GITHUB DISCUSSIONS/ISSUES

You probably installed Audacious incorrectly. What is the output of which audacious ? And pkg-config --libs audacious ?

I use this packs:

GTK+ 3:
audacious-3.6.2-gtk3.tar.bz2
audacious-plugins-3.6.2-gtk3.tar.bz2

This is output of audacious-3.6.2-gtk3 configuration:

@checking build system type... x86_64-unknown-linux-gnu
checking host system type... x86_64-unknown-linux-gnu
checking target system type... x86_64-unknown-linux-gnu
checking for gcc... gcc
checking whether the C compiler works... yes
checking for C compiler default output file name... a.out
checking for suffix of executables...
checking whether we are cross compiling... no
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether gcc accepts -g... yes
checking for gcc option to accept ISO C89... none needed
checking for g++... g++
checking whether we are using the GNU C++ compiler... yes
checking whether g++ accepts -g... yes
checking how to run the C preprocessor... gcc -E
checking for grep that handles long lines and -e... /bin/grep
checking for egrep... /bin/grep -E
checking for ANSI C header files... yes
checking for sys/types.h... yes
checking for sys/stat.h... yes
checking for stdlib.h... yes
checking for string.h... yes
checking for memory.h... yes
checking for strings.h... yes
checking for inttypes.h... yes
checking for stdint.h... yes
checking for unistd.h... yes
checking whether byte ordering is bigendian... no
checking for special C compiler options needed for large files... no
checking for _FILE_OFFSET_BITS value needed for large files... no
checking for pkg-config... /bin/pkg-config
checking pkg-config is at least version 0.9.0... yes
checking operating system type... Linux
checking whether the C/C++ compiler supports -Wtype-limits... yes
checking whether the C++ compiler supports -Woverloaded-virtual... yes
checking whether byte ordering is bigendian... (cached) no
checking whether ln -s works... yes
checking for rm... /bin/rm
checking for mv... /bin/mv
checking for cp... /bin/cp
checking for ar... /bin/ar
checking for ranlib... /bin/ranlib
checking for windres... no
checking for library containing pthread_create... -lpthread
checking for GLIB... yes
checking for GMODULE... yes
checking for GTK... yes
checking for GIO... yes
checking for shared library system... GNU
checking for sigwait... yes
checking for /proc/self/exe... yes
checking for ld used by gcc... /bin/ld
checking if the linker (/bin/ld) is GNU ld... yes
checking for shared library run path origin... done
checking for iconv... yes
checking for working iconv... yes
checking for iconv declaration...
extern size_t iconv (iconv_t cd, char * *inbuf, size_t *inbytesleft, char * *outbuf, size_t *outbytesleft);
checking whether make sets $(MAKE)... yes
checking for a BSD-compatible install... /bin/install -c
checking for a thread-safe mkdir -p... /bin/mkdir -p
checking for a sed that does not truncate output... /bin/sed
checking whether NLS is requested... yes
checking for msgfmt... /bin/msgfmt
checking for gmsgfmt... /bin/msgfmt
checking for xgettext... /bin/xgettext
checking for msgmerge... /bin/msgmerge
checking for CFPreferencesCopyAppValue... no
checking for CFLocaleCopyCurrent... no
checking for GNU gettext in libc... yes
checking whether to use NLS... yes
checking where the gettext function comes from... libc
checking for LIBGUESS... no
checking for gdbus-codegen... yes
checking for tput... /bin/tput
configure: creating ./config.status
configure: touching .deps files
config.status: creating audacious.pc
config.status: WARNING: 'audacious.pc.in' seems to ignore the --datarootdir setting
config.status: creating buildsys.mk
config.status: creating extra.mk
config.status: creating man/audtool.1
config.status: creating man/audacious.1
config.status: creating src/libaudcore/audio.h
config.status: creating src/libaudcore/tinylock.h
config.status: creating config.h
config.status: executing po-directories commands

Configuration:

Install path:                           /usr/local

Automatic character code detection:     no
  D-Bus support:                          yes
  GTK+ support:                           yes
  Qt support:                             no
  Valgrind analysis support:              no@

You didn't really answer the question, but one thing you can try is setting PKG_CONFIG_PATH=/usr/local/lib/pkgconfig before configuring audacious-plugins.

Given that it's x86_64 multilib, it would be /usr/local/lib64/pkgconfig for the arch-specific .pc files installed into prefix=/usr/local.