Comment 12 for bug 1325282

I found the root cause and wrote it in comments 21 and 22 of the duplicate bug:

           https://bugs.launchpad.net/ubuntu/+source/ubuntu-drivers-common/+bug/1296425

SUMMARY: ubuntu-drivers-common scripts (sl-modem.py) are calling "aplay -l" with an empty environment, thus "aplay -l" creates a new "pulseaudio" process instead of using the session's pulseaudio, which interferes with sound card recognition (device busy errors). There is a one liner fix in the comment. Kubuntu calls ubuntu-drivers-common scripts on login to KDE, but the issue affects also Ubuntu users if they run any program that calls those scripts, until they restart.

Cheers,
Juan Manuel Cabo