Default [Q] D802 : sound mute /unmute without action

hello
my wife is using a D802 , stock Rom with lasts OTA updates. ( the last one solved some speaker problem ).

- while using skype, phone switch in mute mode after a few minutes. she have to unmute it again and again.
- switching phone in mute mode for the night ( with ou without skype connection ), phone unmute each day at 7H25. why this time ?, she never set something with this time.

This started to happen few days after the last OTA update, but maybe some applications where updated at same time too.

Have somebody a clue or a direction to search for ? I'm thinking about "sound profile", and "silent hours" but I don't find this options in setup, although phone is updated with kitkat