Audio engine changes:

-Idle tick is now responsible for dispatching all pending sound sources, as such improves priority sorting.
-Added ui sound precache mechanisms (to fetch sounds without associated soundsources)
-Audio engine no logner available pre-login or upon disconnect, for simplicity.
-General cleanup to the audio engine/impl.
-Fixed some gAudiop calls that were lacking prerequisite null checks.
-Added 'isUpstreamOK' procuedure to LLAssetStorage to allow avoidance of making requests when there is no upstream host connection.

indra/newview/llfloaterexploresounds.cpp

@@ -304,6 +304,7 @@ void LLFloaterExploreSounds::handle_play_locally(void* user_data)
 		if(std::find(asset_list.begin(), asset_list.end(), item.mAssetID) == asset_list.end())
 		{
 			asset_list.push_back(item.mAssetID);
+			if(gAudiop)
 			gAudiop->triggerSound(item.mAssetID, LLUUID::null, 1.0f, LLAudioEngine::AUDIO_TYPE_UI);
 		}
 	}