The pull down menu disappears when the computer doesn't recognize the speakers. Thus, the audio cable to your stereo could be loose or Airport could be having a problem. A few months ago I noticed that my browser seemed to be taking forever to load. Well, during those times I noticed that my Airtunes pulldown menu was also absent, so I locailized the problem to somewhere in my Airport system (card, cable, etc). After reseating my Airport card, the problem has all but disappeared. I would recommend at least trying to restart your system before jumping to jiggling with the Airport card, but that isn't too hard either.