ash-enable-system-sounds is exposed to about:flags.

chrome/browser/about_flags.cc

Index: chrome/browser/about_flags.cc
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index b91346dc1892892724a655fe5b312ca0edf1fd09..b38a4d74306a0347904fda6867f6c69784ef1b7c 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -1961,7 +1961,16 @@ const Experiment kExperiments[] = {
     IDS_FLAGS_EMBED_FLASH_FULLSCREEN_DESCRIPTION,
     kOsDesktop,
     SINGLE_VALUE_TYPE(switches::kEmbedFlashFullscreen)
+  },
+#if defined(USE_ASH) && defined(OS_CHROMEOS)
+  {
+    "enable-system-sounds",
+    IDS_FLAGS_ENABLE_SYSTEM_SOUNDS_NAME,
+    IDS_FLAGS_ENABLE_SYSTEM_SOUNDS_DESCRIPTION,
+    kOsCrOS,
+    SINGLE_VALUE_TYPE(ash::switches::kAshEnableSystemSounds)
   }
+#endif
 };
 
 const Experiment* experiments = kExperiments;