Inconsistency with Audio behaviour between Windows and OS X

Working on Unreal Engine for OS X I’ve extended the SunTemple demo by adding multiple audio volumes and reverb effects to add sound to the scene.

Problem is, when I leave an audio volume, the reverb stops dead. There is no gradual decay. I’ve tweaked every setting I can find and it doesn’t seem to help.

I moved the project over to my windows machine and it works fine. When I leave an audio volume the reverb slowly decays as expected.

Any idea what is causing this inconsistency in behaviour? I changed nothing and it worked fine on Windows, just not on OS X.

Thanks in advance for any help.