Split Stereo To Mono
ACX AudioBook suggests delivery in Mono (one blue wave) and some of the sound tests don't work in Stereo.
There are only very rare conditions where you are required to deliver a voice track in Stereo (two blue waves).
There is a drop-down menu under the tiny black arrow to the left of the track.
Split Stereo to Mono.
[X] Delete one of the two now separated tracks.