When I drop audio files into the timeline and play it back, it sounds like a reverb has been added to it. I am using version 18.07.02. Once it is exported, it sounds fine. However, it makes editing more difficult and grating. The audio file is coded as M4A if that helps.
Not happening here.
Can you check your Audio driver properties?
Reverb has not been added. You might be hearing preview stutter.
It appears to happen when adding a second video track with an audio track. So, when there are 2 things running in 2 video tracks plus one audio track, I have preview stutter. I did not have that occur in the previous version.
There has been an audio preview stutter regression in v18.07 that has been mentioned here in quite a few threads. It is fixed for v18.08, which is coming August 1.