Thanks, Michael ! Your note help a lot to my ears.

2009/9/8 Michael Cronenworth <mike@cchtml.com>
It might be that new "feature" implemented in F11 called "flat volumes." In an attempt to copy Windows, for only God knows why, the PA folks made your apps change the system volume. This, coupled with a bug in gstreamer adjusting at logarithmic amounts instead of linear, it makes volume management a pain. The fix?

/etc/pulse/daemon.conf
Uncomment "flat-volumes = yes" and change it to "no" and save. Log out, wait 30 seconds for pulse to die (another great "feature") and log in. Volume level changes are now normal again! Coupled with the latest PA update yesterday, F11 now sounds like F10. Ah...

Have a good one.