I got it.
It's a Dopus bug. When path exceeds from 60 characters long, it does not play the file anymore.
Just create this folder:
and move the wav file up and down to find the limit.
Althought it is clear, for any casual reader just let me point out that in everyday work it is esy to find longer paths, so please do not show me the non-sense of the the path I wrote above.
The down side for me is that there's no solution as long as I decided to stick with version Dopus v8, and any patch would come for v9
Anyway, if you can reproduce the problem I'll report it to deveopment, just another contribution on my side for the common good.