Bad sound if I'm watching movies

[Renecek]

Hello, for last 2 weeks I have a problem with audio. For example:

I'm watching some movie or TV serie from my harddisk or from online website. Audio is OK but after 10-15 minutes audio faded out and it continues very silent.

For vision:
If you plug some headphones into macbook, you set volume to higher and if you put headphones on the table, you hear sound very quietly. I hear movie/tv serie same after 10-15 minutes and I don't have connected any headphones.

Can anybody help me please? I tried to format disk and clean install mountain lion but it didn't help me.
BTW I'm using Logic Pro with external audio device Focusrite Scarlett 2i2 but if I watching movies I don't have connected audio device into my macbook.

MacBook Pro (13-inch, Early 2011)
Original HDD + Intel 520Series joined to Macintosh Fusion
Mountain Lion 10.8.4

Please help me. Thank you very much!

 

[dastinger]

You did check Audio Prefs, right? Sometimes the balance moves out of center (don't know why) and it may give you the impression that all the audio is fading out while it's only one of the channels. If you did and no one here can help you, I'd take it in to an Apple Store.

 

[Renecek]

I have balance at center.. :/

 

[ricede]

Try resetting PRAM

http://support.apple.com/kb/HT1379

 

[Renecek]

already tried, it didn't help me

 

[Renecek]

no help please?