Thanks but I have already done these. May be I wasn't clear the first time - I have a working sound card. I just wanted to know is there a way to make the music sound better.
After I've written the post I suddenly realized that may be there was actually no problem with my sound. As it was supposed to output 7.1 channel surround sound it was clear the sound had to be split in several parts. And using headphones plugged in where the "Front left&right" speakers were supposed to be, I got just the part for the front left&right channel.
That is my conclusion as a non expert in sound distribution. May be there is a way to use this plug as a real stereo output for my headphones but I don't know it.
Thanks again for the help