I tried to replicate the problem, but it does not occur.
I created a new 4.12.5 first person template.
I imported the 14 second flamethrower audio loop sound wave file in question.
I added audio component to first person character, and triggered audio to play when ‘fire’ is pressed, and stopped when ‘fire’ is released.
And disabled auto activate just for good measure.
Everything works fine.
I then opened 4.13.1 and converted the project, tested to try and replicate the problem. The problem does not occur.
I could not replicate the issue in a blank project :\
I then went back to a 4.11 version of my main project to see if the problem still existed, but there’s no problems.
I then converted the project to 4.13.1 and the problem returns again. 
Re-importing the audio doesn’t fix the problem.
I also tried to migrate the audio from the 4.11 project across to 4.13.1 version, and still, the problem persists.
I also just placed the sound cue into the level itself, using various lengthy sound waves, and the same problem occurred. (I thought maybe it was the audio cue and concatenator causing a problem.)
I should also point out, the audio files are in 44100khz, and the original files are .wav files when I import them if that helps.
I’m a little bit stumped by this to say the least :\
This is consistent with a range of issues when converting my project which I couldn’t possibly summarize.
In fact I think I spend more time fixing conversion problems (such as conversion deleting event nodes, corrupting blueprints entirely, resetting variables to default values etc) than actually working on my game content
Is there a better way of converting to a new version other than opening the new engine version and selecting my older project to convert, because it’s really hindering my development massively to the point where I’m considering just throwing in the towel after almost 2 years of work