I believe most people who have experienced the power (should I say pain?) of the Intel HDA chipset know what I am talking about. It starts nicely, you play one file (audio or video) and the chipset behaves (almost) nicely in ALSA. Before you know it, you try mixing sounds and playing a second file at the same time. This is when you are greeted with a nice message such as alsa-lib: pcm_dmix.c:996:(snd_pcm_dmix_open) unable to open slave.

It took me a while to figure out but I eventually found a thread and I believe that since It has worked for a few people before me (and it does for me too), then it might just work for others.

So, you can put the following in your ~/.asoundrc :

pcm.!default {
        type plug
        slave.pcm "swmixer"
}

pcm.swmixer {
        type dmix
        ipc_key 1234
        slave {
                pcm "hw:0,0"
                period_time 0
                period_size 1024
                buffer_size 4096
                rate 44100
        }
}
ctl.swmixer {
    type hw
    card 0
}