Hi! Another hint:

In Linux Mint 7 / ubuntu jaunty, I compiled (configure and make only) the alsa-plugins that I got from the alsa project page.
(First I installed libasound2-dev from the ubuntu repos).

Then I copied these two files (the .so alone was not enough) to /usr/lib/alsa-lib:
.../jack/libasound_module_pcm_jack.la
.../jack/.libs/libasound_module_pcm_jack.so

I created the .asoundrc file with this content:

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

    pcm.jack {
        type jack
        playback_ports {
            0 system:playback_1
            1 system:playback_2
        }
    }

It works for me to hear to youtube videos, jamendo and the like.

See

https://bugs.launchpad.net/ubuntu/+source/alsa-plugins/+bug/84900

Cheers, Pablo