What software will multiplex sound inputs?

<p>My computer has one of those crappy sound cards that only takes input from one source. I run fvwm2. I play music all the time. If I browse to a website that plays sounds, like YouTube, then no sound comes out from my browser, and then Firefox locks up tight. I have to kill the individual firefox processes and restart it.</p> <p>KDE and (I think) Gnome have some lowlevel software that acts as a sound multiplexor. It sits above the crappy sound card and allows multiple sound inputs. What is this software? Can I use it without using KDE?</p> <p>EDIT: At the low end, I am using ALSA for sound. At any given time, there are only two programs that are going to play sound - one is mplayer and one is Firefox. I think both these programs are trying to get sole ownership of /dev/dsp to play sound. Are there settings that will cause mplayer and Firefox to pass the sound to some underlying device that will multiplex the sound? Arts? Esound? PulseAudio?</p> <p>Firefox is using mplayer as a plugin to play audio. Is there a way to configure the mplayer plugin so it doesn't go directly to /dev/dsp but to the multiplexor?And is there some configuration for mplayer on the command line that would pass the sound to the same thing?</p>