No acoustic tones - after car went to sleep

[slovakE70]

Hello,
I have bmw x5 e70, 50i N63 engine. My problem is that everytime i start the car in the morning, i get "No acoustic tones" message on i-drive and my sound doesn't work. BUT when i shut the car off and locked doors, then after i unlock and start car again in like couple minutes, sound starts working. Everytime its like that.
Theres are no codes on ODB scan.
When sound is working and i lock the car for longer period (lets say 30 or more minutes), sound will not work on start and "no acoustic tones" message will pop up.
It appears that everytime car goes to sleep, on the next start sound will not work - no acoustic tones message. During the day when im not let the car locked for more then 20 minutes, sound will work normally on every start-up.
BMW servise in my country said that the problem is in the amplifier. This should be the Top-hifi system amplifier. But my question is - if the amplifier was bad, the sound should not work all the time ?
When i want sound to work in the morning (or after longer time that car was locked), i simply shut ignition off and lock the car for 1 minute and after that i unlock, start the car and sound works.
Does annybody have similar problem ?