Sound output to airplay device time out...

I have selected airplay speakers as my prefered sound output so I can stream real time audio to speakers connected to my airport express. There is no problem in playing audio via itunes as the airplay speakers / airport express is alway selectable, however I can only select other audio sources to play (spotify, BBC iplayer...) through the airplay port after a computer restart, once the computer (macbook pro - running mountain lion 10.8.1) goes to sleep and wakes up the airplay audio output is no longer selectable. Its a bit of a hassel to have to re-start the laptop everytime to select the audio output option. Obviously a software glitch!