I noticed that whenever I enter those shadow portals in Clockwork city, the volume of the sound effects in the game afterwards tend to decrease.
For example, starting the game normally (before entering the portal); When I ride the mound and sprint, the shout of my toon seem loud enough. But after I enter into or exit from a shadow portal, and then ride my mount and click sprint, the loudness of my toon's shout is decreased. Thats just one example.