Comment 24 for bug 84900

It's actually the jack/.libs/libasound_module_pcm_jack.so file that gets moved to /usr/lib/alsa-lib

The rest of the details can be found by searching for keywords "asoundrc jack" which will bring you to this page:

http://www.alsa-project.org/main/index.php/Asoundrc

Anyway, this was the critical text that goes into your ~/.asoundrc file:

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

pcm.jack {

    type jack

    playback_ports {
        0 system:playback_1
        1 system:playback_2
    }

I've had excellent success with this just now (Ubuntu 9.10, compiled from alsa-plugins 1.0.18), and actually have a firefox browser playing a myspace music page to headphones connected to a remote machine (using jack_netsource, jackd -d net, and jack_connect to assign ports). You don't need any of that if you plan to do everything on your local machine. But it's pretty clear to me at least that all of this stuff starts to get into more advanced and tricky territory, and from a support standpoint I can see why the Ubuntu folks would opt to keep a new user from venturing into _any_ of this territory, when just trying to get rudimentary sound output working.