AAC tagging seems to use AtomicParsley. However that was not defined in the script and must be set using "ATOMICPARSLEY=AtomicParsley" (Filed bug upstream). When it is defined and AtomicParsley is available, AAC tagging works as expected. Ebuild should add that as a dependency for aac useflag.
Fixed in CVS, thanks for reporting. :)
reopening, missing fix in portage
Closing again since this is fixed in abcde-2.4.2-r1.