More audio improvements

- Fix a bug that made the last lap SFX played only if music was on.
- Fix the behavior of the music volume adjustment when the last lap SFX is played. Previously, the music volume was only adjusted when the music volume was above a certain threshold, but when it was above that threshold, the adjustment was too strong.

src/modes/linear_world.cpp

@@ -421,17 +421,20 @@ void LinearWorld::newLap(unsigned int kart_index)
         }
         if(!m_last_lap_sfx_played && lap_count > 1)
         {
-            if (UserConfigParams::m_music)
+            if (UserConfigParams::m_sfx)
             {
                 m_last_lap_sfx->play();
                 m_last_lap_sfx_played = true;
                 m_last_lap_sfx_playing = true;
 
-                // In case that no music is defined
-                if(music_manager->getCurrentMusic() &&
-                    music_manager->getMasterMusicVolume() > 0.2f)
+                // Temporarily reduce the volume of the main music
+                // So that the last lap SFX can be heard
+                if(UserConfigParams::m_music &&
+                    music_manager->getCurrentMusic())
                 {
-                    music_manager->setTemporaryVolume(0.2f);
+                    // The parameter taken by SetTemporaryVolume is a factor
+                    // that gets multiplied with the master volume
+                    music_manager->setTemporaryVolume(0.5f);
                 }
             }
             else

src/network/protocols/client_lobby.cpp

@@ -1686,6 +1686,7 @@ void ClientLobby::handleClientCommand(const std::string& cmd)
         AddonsPack::install(argv[1]);
     else if (argv[0] == "uninstalladdon" && argv.size() == 2)
         AddonsPack::uninstall(argv[1]);
+    // FIXME - this code duplicates functions that should be handled elsewhere.
     else if (argv[0] == "music" && argv.size() == 2)
     {
         int vol = atoi(argv[1].c_str());