I use pulseaudio 2.1-r1.
As long as no sound is played using flash player on youtube which might use any ALSA wrapper not directly pulseaudio (am I wrong?) sound is muted if volume level is under 35 % for my USB headset.
When I set it to 35 % it has to volume you could expect for 35 % (it's quite loude).
When I watch a video on youtube and the ALSA plugin [plugin-container] playback stream is displayed the whole volume becomes louder (but the value stays at 35 %) AND I am able to set the volume below 35 % how it is supposed to be (its not muted anymore, the volume just gets lower as expected).

This only occurs when using the USB headset not with "Built-in Audi Analog Stereo" output.

Some version details:
alsa-lib 1.0.26-r1
kernel 3.5.7-gentoo


/etc/asound.conf:
pcm.pulse {
    type pulse
}
ctl.pulse {
    type pulse
}

pcm.!default {
        type pulse
}

ctl.!default {
 type pulse
}


This is my USB device:
Bus 001 Device 012: ID 0d8c:000c C-Media Electronics, Inc. Audio Adapter
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               1.10
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0 
  bDeviceProtocol         0 
  bMaxPacketSize0        64
  idVendor           0x0d8c C-Media Electronics, Inc.
  idProduct          0x000c Audio Adapter
  bcdDevice            1.00
  iManufacturer           0 
  iProduct                1 C-Media USB Headphone Set  
  iSerial                 0 
  bNumConfigurations      1