Well, assuming these aren’t protected, like iTunes aac files, then you can convert the aac to mp3 with foobar2000 and lame mp3 encoder. Here are some basic instructions on using those two (stole these from another site)
[I]The basic idea is you to use the foobar2000’s builtin converter to convert any music file it can play into another format. The first thing you need to do is to tell foobar2000 where the lame executable is on your computer. foorbar2000 does provide a default profile for lame which is probably good enough for you. Once that is done, load the file into foobar2000 either by drag/drop, double-clicking the file. Right click on the file you want to convert, choose the context menu item “convert”, choose “convert to…” and ensure the correct encoding preset is chosen (the one you shoul duse is MP3 (LAME)). Click ok, and you should get a mp3 file in a short time.[/I]
Or you can convert with Nero if you have it. Or SUPER.
The SUPER website is one of the most annoying on the planet…the actual download link is on the third page and they don’t allow direct links…http://www.erightsoft.net/SUPER.html
Another tool to try going straight from the aac files to an audio cd is called Burrrn. http://www.softpedia.com/get/CD-DVD-Tools/Audio-CD-DVD-Burning/Burrrn.shtml I have a sneaking suspicion that you will get the same filter error using Burrrn as you had with ImgBurn though.