The problem I am having is with the sound.
Sometimes it gets through to the TV, so no sound on the MBP itself while air-playing;
but sometimes the sound stays on the MBP.

and the choice seems random (or at least I haven't figured out the pattern).

Anyone experiences the same? 

Mine is a MBPr (fairly new) and the Apple TV is the latest generation with no jail-break;
Both connect via WIFI.