I'm having the same issue. Just got the box several weeks ago. Firmware upgraded to 1.1.3. The problem appears if you try to use Kodi higher than the 15rc1 with passthrough. Audio drops out constantly at an approximately 5 second interval.
I reverted to old Kodi. However, frequent audio dropouts occur over passthrough including stereo signal regardless of player used including the himedia player. These dropouts occur at random... sometimes seconds apart... often minutes apart, rarely 10s of minutes apart.
For both issues I've found numerous posts on various sites but no solutions.