Unfortunately with Macs, if you copy mod files and subfolders into the "minecraft.jar" folder (that you make while installing on a Mac) and "Replace" old files, if there were any subfolders of the same name already in the "minecraft.jar" folder, then it will remove everything from those subfolders and just replace them with the new files.
What this means for you is that, since FML has a "paulscode" subfolder (containing the file "paulscode/sound/codecs/CodecIBXM.class"), everything that was already in "paulscode" goes away - your entire soundsystem.
To get around this, you'll need to copy over everything else from that forge universal zip except for the "paulscode" subfolder, and then go down into the "paulscode/sound/codecs" subfolder and copy "CodecIBXM.class" directly into the "paulscode/sound/codecs" subfolder of your "minecraft.jar". Or else use a mod installer/launcher program like Magic Launcher to avoid touching the "minecraft.jar" at all.
(It's a shame, really... if it weren't for that single "CodecIBXM.class" file, there'd be no problems with installing Forge on a Mac in the usual way.)