Modify the volume visuals:

ash/system/audio/tray_volume.cc

Index: ash/system/audio/tray_volume.cc
diff --git a/ash/system/audio/tray_volume.cc b/ash/system/audio/tray_volume.cc
index b43f1218deff505379c7f30501f6e70e853dd043..869c517c3a470eddfed8b096f8e960c333571ecf 100644
--- a/ash/system/audio/tray_volume.cc
+++ b/ash/system/audio/tray_volume.cc
@@ -106,7 +106,8 @@ class VolumeView : public views::View,
         ash::Shell::GetInstance()->tray_delegate();
     slider_ = new views::Slider(this, views::Slider::HORIZONTAL);
     slider_->set_focus_border_color(kFocusBorderColor);
-    slider_->SetValue(delegate->GetVolumeLevel());
+    slider_->SetValue(
+        delegate->IsAudioMuted() ? 0.0 :  delegate->GetVolumeLevel());
     slider_->SetAccessibleName(
         ui::ResourceBundle::GetSharedInstance().GetLocalizedString(
             IDS_ASH_STATUS_TRAY_VOLUME));
@@ -148,8 +149,13 @@ class VolumeView : public views::View,
                                   float value,
                                   float old_value,
                                   views::SliderChangeReason reason) OVERRIDE {
-    if (reason == views::VALUE_CHANGED_BY_USER)
-      ash::Shell::GetInstance()->tray_delegate()->SetVolumeLevel(value);
+    if (reason == views::VALUE_CHANGED_BY_USER) {
+      ash::SystemTrayDelegate* delegate =
+          ash::Shell::GetInstance()->tray_delegate();
+      delegate->SetVolumeLevel(value);
+      if (delegate->IsAudioMuted() && value > 0.0)
+        delegate->SetAudioMuted(false);

[sadrul, 2012/05/19 02:10:54]

I don't think this is the right place for this change. Volume can be changed by
pressing the corresponding accelerator keys etc., and you will need to duplicate
this same code there too. Instead, the audio should be muted when volume-level
is set to zero in AudioHandler::SetVolumePercent so that we have this logic in
only place.

[Daniel Erat, 2012/05/19 05:38:34]

Sorry, I gave conflicting feedback in the review for
https://codereview.chromium.org/10411028/ (saying that a UX decision like this
one should be implemented in the UI rather than in the lower AudioHandler
layer).  My main concern was that enforcing the policy in AudioHandler removes
the flexibility to change the volume level that we'll restore to on unmute
without actually unmuting.

[Jun Mukai, 2012/05/21 17:05:38]

Still not sure which is better, but is there any real scenario where a user
wants to change the volume but not to unmute?  I couldn't imagine such case, so
sadrul's point sounds slightly more valid to me.

On 2012/05/19 05:38:34, Daniel Erat wrote:

[Jun Mukai, 2012/05/21 17:19:38]

Note that same thing was introduced to volume_controller:
http://src.chromium.org/viewvc/chrome/trunk/src/chrome/browser/ui/views/ash/v...

On 2012/05/21 17:05:38, Jun Mukai wrote:

+    }
     icon_->Update();
   }
 
@@ -171,7 +177,9 @@ TrayVolume::~TrayVolume() {
 }
 
 bool TrayVolume::GetInitialVisibility() {
-  return ash::Shell::GetInstance()->tray_delegate()->IsAudioMuted();
+  ash::SystemTrayDelegate* delegate =
+      ash::Shell::GetInstance()->tray_delegate();
+  return delegate->GetVolumeLevel() == 0.0 || delegate->IsAudioMuted();
 }
 
 views::View* TrayVolume::CreateDefaultView(user::LoginStatus status) {
@@ -201,6 +209,8 @@ void TrayVolume::OnVolumeChanged(float percent) {
     tray_view()->SetVisible(GetInitialVisibility());
 
   if (volume_view_) {
+    if (ash::Shell::GetInstance()->tray_delegate()->IsAudioMuted())
+      percent = 0.0;
     volume_view_->SetVolumeLevel(percent);
     SetDetailedViewCloseDelay(kTrayPopupAutoCloseDelayInSeconds);
     return;