Help. Getting the same sound on all channels instead of 5.1

[PeterTaps]

Folks,

I am running Ubuntu 11.10. The three stereo jacks on the PC are connected to an amplifier.

I have a .mp4 file that contains 5.1 sound. When I play this .mp4 file through VLC, all the six speakers individually work as expected.

However, once in a few days, the same sound starts playing through all the channels.

This is confusing. I am wondering if someone can help me understand what is going wrong.

Thank you in advance for your help.

Regards,
Peter