This is because in the standalone aac version, there is no checking for file type. But in the combined version there is a checking of file type before doing action. Try this version I've attached instead. I added another item that looks for mp4 files. This should work. I can't check as I don't have any mp4 files.
By the way, I'm a member of the old farts club as well. I knew absolutely nothing about all of this stuff 2 years ago, but have learned a tremendous amount just from hanging out at this board (and people helping me out just like I answered your question) and the board over at hydrogenaudio.org and dbpoweramp and squeezebox:
http://www.hydrogenaudio.org/forums/index.php
http://forum.dbpoweramp.com/
http://forums.slimdevices.com/
I essentially use dbpoweramp (ripping to FLAC (home use & archive) and converting to mp3 (mobile use)) > mp3tag (tagging) > foobar2000 (playing/organizing/some utilities) > itunes (syncing with IPOD) or squeezecenter for playing through several squeezeboxes connected to home stereo system.
Just read a lot on these discussion boards and experiment. And always keep backups of your music files, just in case!
ReplayGain_and_ITUNES_Compilation_Combined_MP3___AAC.mta (370 Bytes)