this audacity-wxgtk2 :
https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=audacity-wxgtk2 ?
this is same issue i had here :
https://forum.obarun.org/viewtopic.php?id=1291 (except that the build doesn't fail, just produces a dodgy package)
to do with the mysterious missing cmake option: -DCMAKE_INSTALL_LIBDIR=lib (as stated, some projects specify /usr/lib64 in their source)
the mystery being: why "pure arch" apparently no longer needs to specify this cmake option.. (contrary to the packaging guidelines:
https://wiki.archlinux.org/index.php/CMake_package_guidelines# Prefix_and_library_install_directories )