Small update to sound manager and improved UI readability — excellent Steam update #2293886

This is a short update that aims to fix an error that sometimes would appear on sound playback, in particular the error that referred to the application of filters on sounds. I thought I had already included this fix, but unfortunately I have forgotten.Another important fix is in regards to improved UI on the level select screen. This is about seeing, at a glance, which levels are locked and which are unlocked, without having to hover over them. This is done visually, by having different colors for the locked and unlocked state. Other small UI fixes are also applied.