mpg321 includes libao's ao.m4 (which hardcodes LIBDIR) in its aclocal.m4.
Fixed with mpg321-0.2.12-r1.ebuild: 21 Aug 2011; Matt Turner <mattst88@gentoo.org> +mpg321-0.2.12-r1.ebuild, +files/mpg321-0.2.12-check-for-lround.patch: Check for -lm, since it wasn't added to LIBS. Run autoreconf to generate aclocal.m4 without hardcoded LIBDIR=lib, bug 380027.