They are not mono files, they are stereo files both those that include the nightly build and those included in the stock version. With any other stock rom and other roms the sounds are reproduced perfectly by the phone loudspeaker when the headphones are connected, in this rom does not happen like this, it's more, if you disconnect the headphones when a ringtone sounds the two audio channels are played by the phone speaker. YES IS A BUG. Try for example with ICS stock ROM and any of its sounds, i recommend the Signature sound. Then try this same sound with this rom.