I have no idea if this will help anybody but here goes nothing:
1. Playing a movie on apple TV via home sharing
If the sound is low or non existent do this:
1. Menu => settings => airplay => select airplay speakers
2. While playing the movie hold the OK button pressed until a menu appears
Select sound
Select the airplay speakers and use the right button on the remote to increase volume
2. Streaming a movie from your computer
If for example you are using iTunes to stream you will see that the volume control in iTunes is blocked. You dont need it
The fix for this is identical to the above one
This helped me. Hope it will help you also!
Cheers
My System:
Airport Express
Bose Speakers connected to the Airport Express
Apple TV 3rd gen
Samsung S24B370 monitor
MBP E2011