On Thu, 2011-05-26 at 00:49 -0500, Manuel Escudero wrote:
it doesn't matter wich device I choose as output, the PC simple don't mute one or another and my headphones and speakers sound at the same time,
I recall, on one sound card, going through the preference available in the volume controls, and finding a "detect headphones" switch. When enabled, that sound card would notice when headphones were plugged in.
That was on a (much) older Fedora release, but it's probably still something to look for.