By default, AAC support is disabled in the Debian mythmusic package. There used to be a libmp4ff distributed with the libfaad* packages, but upstream's makefile no longer installs it so it looks like it got dropped out of the debs. To get AAC support working again, I had to rebuild the faad2 source package with the following patch to debian/rules (download):
--- rules.orig 2007-02-03 18:52:28.000000000 -0500
+++ rules 2007-02-03 17:07:39.000000000 -0500
@@ -61,6 +61,10 @@
dh_installdirs
$(MAKE) install DESTDIR=$(CURDIR)/debian/${version}
+ cp common/mp4ff/.libs/libmp4ff.a $(CURDIR)/debian/${version}/usr/lib
+ cp common/mp4ff/libmp4ff.la $(CURDIR)/debian/${version}/usr/lib
+ cp common/mp4ff/mp4ff.h $(CURDIR)/debian/${version}/usr/include
+ cp common/mp4ff/mp4ff_int_types.h $(CURDIR)/debian/${version}/usr/include
rm debian/${version}/usr/lib/xmms/Input/libmp4.a
rm debian/${version}/usr/lib/xmms/Input/libmp4.la
Update 2/4/07: I've filed Debian Bug report #409648 on this.
