snd-aoa is a driver for the sound on newer Apple machines. It currently covers all machines with a layout-id property in the sound node in the device tree, among others
However, it does not do any digital input yet.
snd-aoa was included into linux as of 2.6.18-rc1 and support for machines with a layout-id property was removed from snd-powermac, hence it no longer conflicts. The information on this page is historic.
git clone http://git.sipsolutions.net/snd-aoa.git
only the bus module i2sbus should need manual loading, all the rest should auto-load provided the modules were installed by make install. At boot, i2sbus loads automatically due to keying off the macio device, hence no modules.conf entry is needed.
Of course snd-aoa is not compatible with snd-powermac, please remove snd-powermac. Don't load it at boot (i.e. remove from your modules.conf) or best even don't compile it into your kernel
Please see the FAQ in the source distribution: http://johannes.sipsolutions.net/gitweb.cgi?p=snd-aoa.git;a=blob;f=doc/FAQ