Hi there, I need help to configure Ubuntustudio on my machine

[Pablo Fernandez]

I am confused. Those messages "unknown PCM default" / "unknown PC
Juli"... This looks like an alsa problem, i.e., at a lower level than
jack or pulseaudio.

I would try disabling .asoundrc files [1]. There might be two of them:

/etc/asound.conf  (probably not existent, but just in case)
~/.asoundrc

The second one, from your report:

# ALSA library configuration file
# Include settings that are under the control of asoundconf(1).
# (To disable these settings, comment out this line.)
</home/matthias/.asoundrc.asoundconf>

So, what happens if you comment out the last line? (And then reboot,
or restart the user session)

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