You were very generous with me
(Have you checked whether this app has an option in its settings to import mp3s or use user-provided files in this way? Does it create a folder in the user-accessible part of the phone's storage (e.g. /sdcard/WhateverTheAppIsCalled)? If it does you could try putting your files there. This would be the simplest solution.
If not, then to be honest you are out of luck. You'd need to root your device in order to copy these mp3 files into its internal folders, and even if you did that there's no guarantee that the app would use them.)
No.
I simply wanted its mp3 content
Please answer the following question only by YES or NO in order not to burden you.
Is it safe to root my device?
And is it easy and can be done by every one? and not only by specialist ?
Will rooting certainly enable me to see the apps file?
Thank you
Oh, you want to extract a file from it - I thought you meant you wanted to add a file to it.
(Of course unless the app tells you the files are mp3 they may be another format, but changing formats can't be too hard).
So to answer your questions (I'm afraid there aren't yes/no answers to them):
* Is it safe to root my device?
We'd need to know what device you have in order to say whether it's even possible to root it. Probably best to tell us what country the device was intended for as well, since that can matter (e.g. a European Samsung s10 can be rooted, a US one can't). And what Android version is it running may matter. It's safe provided you follow instructions precisely, and don't try to install software written for a different phone model on your phone (no matter how similar that model, some tools have to be built for exactly your model of phone).
* And is it easy and can be done by every one? and not only by specialist?
Assuming it is possible, the method depends on what device you have. It typically requires a computer, a couple of downloads, a couple of command line instructions on the computer, and following some simple instructions on the phone. At least one of the downloads must be made specifically for your model of phone (not any other, no matter how similar). The process also usuallly wipes everything off the phone, so backing up first is important. There used to be "one click rooting apps" but they only work on very old versions of Android and frankly I never trusted most of them not to do something I wouldn't want.
* Will rooting certainly enable me to see the apps file?
If you use a file explorer that's capable of using root access then yes, once you can give it root permissions you will be able to look at the app's resources.