sound problem on mini cooper S 2011

[winneratall]

I have my mini cooper S 2011 since Dec 2010. but since my wife drives most of the time, she didn't tell me all the problem she has with the car until I experience it.
I understand it kind of difficult to troubleshoot since it randomly happens. Ok. here is the problem.
in the morning when I start my car, the radio is on, which you can change the channels from the controller. but there is no a sound come out from the speaker. so I test with my ipod, CD player. they all have no sound. I turn off the car and restart it still has no sound. but then after I got off from work. I start my car. the sound come back. I try to reproduce the problem. but I can't. and it is randomly happen. I took to the dealer ship to check. but they can't find the problem. as I say it was random.
for example.
there was once put the key in, and I am sure the radio was on. but no sound even after I start the car, turn off and restart. still no sound.turn the radio off and on. still no sound.
there was once, when I put the key in. there was radio sound(my radio was on) but when I start my car, there was no sound. even the radio still on. and I turn it off and turn on, still no sound.
there was another once, after I use my blue tooth for talk on the phone. after hang off the phone. the radio was on. but no sound.
there are a lot more examples. but none of them can use to reproduce the problem. as I left my car in dealer ship to check. but they can't find any problem.
what should I do? please help.