Many thanks for your quick answers!
I have tried some of your ideas, but unfortunetly the problem is still there.
In my /etc/pulse/daemon.conf, I can find this line: resample-method = speex-float-1
I guess this is what you suspect; correct?
Here it the full content (I have not configured anything more default configuration) with only non commented lines:
resample-method = speex-float-1
flat-volumes = no
default-fragments = 8
default-fragment-size-msec = 10
Commenting the first line ("resample-method = speex-float-1") improves a little (note that for having the configuration being taken into account, I have restarted the pulseaudio daemon with such a command which terminates the daemon: pulseaudio -k. The init process (pid=1) automatically restarts it on my ubuntu 11.10).
I have tried a "test web page"; with the change, the sound becomes choppy after a _longer_ time of "scrolling": around 10 seconds (no resample option) instead of around 5 seconds (with option). Moreover, once the sound has chopped one time, if I scroll again the sound is choppy immediatly. If I wait a long time (about 1 minute), I need to scroll again 10s or 5s ("respectively") to get the choppy sound again. As if a sound buffer in the system were longer or were less used with smaller resampling; nothing else, the problem is still there, just delayed.
But with another "test web page", I can not notice any improvement. So this is not very reproductible... except the choppy sound!
I do not have any USB hub. The PC is a Dell Latitude D820, CPU is a Core2 duo, @1GHz (3990 bogomips per cpu in /proc/cpuinfo). I tried the 4 mother board ports and the problem is the same for all of them. The track sound comes from the motherboard hard disk (no SSD, no NAS, etc); the disk is a relatively high performances one; this is not the original one, it is a WD Scorpio Black 7200trs/mn (WD3200BJKT) - able to run at more than 100Gbps.
So to my mind the issue does not come from the source, all the more that when it occurs, the disk led activity does not hit (all the music is probably already in the 2GB of RAM!).
My global feeling is that something like a wrong priority setting is chopping the audio flow _around the USB_, not in pulseaudio or in mplayer (or VLC btw) because 1) if I change priority of them, nothing is improved and 2) if I used the internal sound card of my PC (not the AMB y2) then there is no choppy sound (in the same conditions).
I suggest to use the PC sound card instead of the y2 since it is not chopping the sound... I'm just kidding
Any other ideas?