I am using a MacPro 13" Retina running OSX 11.6, El Capitan. This is the same configuration that was used to record using a recorder switching app Simple Recorder. I recorded several hours of a conference to transcribe minutes of the meeting.
All of the files recorded as m4a files, and play back clearly on Quicktime, 5KPlayer, and others; so the audio is there. The files load into ExpressScribe and appear to be playing, as the counter is proceeding at a normal rate. However, there is no audio, at any volume. I have tried converting to aiff, wav, and mp3 with the same results. ExpressScribe's "welcome" file plays and the audio is loud and clear.
I purchased and downloaded the newest version of ExpressScribe, hoping that would help. It did not...problem persists.
It sure would be nice if such a vitally important tool, had a bit of real free support...at least during the set up and configuration time.