I plan to buy a BeoSound 5 with BeoMaster 5.  The BM5 will go hidden in a closed cabinet.

The dimensions of the BM5 are indicated to be 38x28x10 cm (WxDxH).  But there is no information about the required free space after the device, for cables and connectors.  In other words, how deep should my cabinet be to accomodate a BM5 with connected cables ?

I'd say the cables would add less than five centimeters (two inches). However, you'd need at least 15-20cm behind that box if you intend to be able to check cables and/or swap them.

I don't have one, but cables are the same, no matter the box.

It might be possible to mount it vertically, thereby having the cables at the bottom, and therefore no ekstra space behind the thing.