I'm confused, First you said (the sound of any plain disappeared ,is missing for a few seconds ,and reappared after few second)
Now your saying (i have this problem just inside the cockpit , with merlin engine, hmm for example p 51 and p 40 f ,all other worked perfectly)
I will try and help anyone problem solve their sound issues when the relation is between my mod and VPmod. I say this because there is no P40F in the VPmod, unless I have missed something in my install
I see a few posts back you say (confirmed in 4.13.3 modact 6.3). Is it in this version your having issues, if so all of my advice would be guessing as I have no idea how you have your sound mod setup. eg: are you using IL2 custom sound folders, a sound mod in the #SAS folder, sounds from within SFS files etc.
All that said my first guess would be to look into the motor.Rolls-Royce-Merlin.prs file being loaded in your game. Either values written inside it are not correct or maybe there's a problem with the wav files that it is calling for.
Cheers DaveB
PS: I did test the merlin in my install and I am not having a sound drop out issue