You may have noticed on a stand-alone Linux server (no GUI) that only one device at a time can use the sound card.  This is deeply annoying and frustrating.  It turns out you don't need to mess around with Pulse, Jack or OSS4 to get different sound input streams working, you can use ALSA, but its default config needs tweaking.

The problem is that your ALSA loving sound programs may latch on to the "default" device, which doesn't multiplex.  The first program to open the default device locks it.  What we need to use is the dmix ALSA plugin.  

So we set up the DSP device all these apps are using to secretly pass everything through to DMIX:

File /etc/asound.conf:

pcm.dsp {    
    type plug    
    slave.pcm "dmix"


And that's it!  Restart all your sound apps (in my case MPD and VLC/sdl) and they should now play together smoothly.

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multiplex audio