Iceweasel, flash and usb headset

[Marco Vittorini Orgeas <marco@mavior.eu>]

Hello,
my problem is the follow:
I am trying to enjoy flash audio contents through my Logitech usb headset on Lenny.
First some console output:

## > cat /proc/asound/cards
##
## 0 [Intel          ]: HDA-Intel - HDA Intel
##                      HDA Intel at 0xfe220000 irq 17
## 1 [Headset        ]: USB-Audio - Logitech USB Headset
##                      Logitech Logitech USB Headset at usb-0000:00:1a.1-1, full speed


## > asoundconf list
##
## Names of available sound cards:
## Intel
## Headset

I've installed adobe flash plugin for iceweasel not through the packaged version (flashplugin-nonfree) but simply putting the .so file object that can be found here http://labs.adobe.com/downloads/flashplayer10.html for 64 bit systems because quite a good number of persons report it to be working without problems.And in fact it is.

Then, the problem: if I plug in my usb-headeset and I go to preferences->sound and for "Music and movies" i choose as sound playback "USB audio",
 Totem for example honors the fact I've choosen the usb as output and it outputs sounds through the headset.

If I try to access  some flash videos through iceweasel, I get the the videos played and the sound always go through the HDA-Intel sound card!
I tried also to do something like: "asoundconf set-default-card Headset" and tried to restart iceweasel but nothing...the sound is always outputted through the Intel pci sound card.

I tried starting iceweasel from command line to get some debug stuff:
when runned from my user I get no relevant messagges, but when I run it from a root shell session,I instead get no sound at all and some messages:

ALSA lib pcm.c:2144:(snd_pcm_open_noupdate) Unknown PCM default
ALSA lib confmisc.c:1286:(snd_func_refer) Unable to find definition 'defaults.namehint.extended'
ALSA lib conf.c:3513:(_snd_config_evaluate) function snd_func_refer returned error: No such file or directory
ALSA lib conf.c:3985:(snd_config_expand) Evaluate error: No such file or directory

I don't know actually what to look for,and my research give me no point to start with.

I think the problem may be related to how actually iceweasel access the sound devices...any suggestions about that would be really appreciated.

As a bonus question: what is the proper way to set the default sound-card and the priority of the sound-cards which could be plugged in the system? 
I mean that for now the << preferences->sound and for "Music and movies" choosing as sound playback "USB audio">> permit the system to fall-back to the hda-intel interface if I un-plug the headset, but what if I had more than two interfaces ?

thank you