If you want to replace the music files in your AChat installation try the following:
1. Got to the main AChat directory and there open the sounds/music directory
2. The .ogg files are the music files. You can simply replace the ogg files with your own files but keeping the original file names, or add your files and edit the ogg_playlist.txt file according to your needs.
IMPORTANT!
The ogg files you use should have 22KHz sample rate.
I used 44.1Khz sample rate ogg without trouble, but you can't use space in file names.