Apologies for the (very) late reply on this but I’ve just come across the same problem and think I’ve narrowed down the possible causes…
Firstly, to answer your question I’m using engine version 4.23. With my current project, I’ve tried this with the audio mixer enabled and disabled and am getting the same results. However, I think this may be related to an issue with virtualizing a sound when silent rather than the adjust volume function. I’ve got a sound (a looping, continuous piece of music) attached to an AudioComponent that I adjust to (volume) 0 over 3 seconds using the AdjustVolume function. I then attempt to bring the volume back up to 1.0 over 3 seconds but nothing can be heard. To test whether the same behaviour occurred without reducing the sound to 0 I repeated the setup, but this time reduced the volume in the first AdjustVolume function to 0.1. This worked, and the second AdjustVolume function raised the volume back to 1.0 over 3 seconds as expected.
I double checked that Play when Silent was selected in the virtualization properties of the attached audio clip. I’m guessing this may be the root of the problem but I’ve no idea how to work around this other than not using the sound virtualisation feature at all and instead just working around the issue by reducing the volume to something inaudible such as 0.0001.
Has anybody else experienced similar issues? And more crucially, can anyone please provide a solution? 
Many thanks,
Joe