audio cut in the middle after exporting video in premier pro

I am having this kind of problem since few weeks ago,

(or maybe it had happened before but I did not notice)

I was doing a very simple job with the premier pro

1. import video (about 10min long)

2. put title

3. cut the video

4. export

this process takes less than 5 minutes

Now the thing is that 

1st export : audio was muted after 10sec (first 10 sec were literally fine)

2nd export : audio was muted after 7min

few minutes later export : audio was all okay

after 1hour export : all audio was okay

none muted

it is weird that I have to wait few minutes (or hours I guess if video was longer)

to get all of my audio exported.

I can assure you that

i haven't changed anything,

any option in exporting or the video itself

I first thought it could the render problem

but, the bar in the timeline was all yellow

and there wasn't anything to render