Amazon's MP3 Downloader is released only for 32-bit systems. It requires libglademm-2.4.so.1. app-emulation/emul-linux-x86-gtkmmlibs does not include libglademm as of the 20110928 version.
Looks like this is fixed with the latest emul set: $ qfile -v libglademm-2.4.so dev-cpp/libglademm-2.6.7 (/usr/lib64/libglademm-2.4.so) app-emulation/emul-linux-x86-gtkmmlibs-20120127 (/usr/lib32/libglademm-2.4.so)