Access to libs path in application package

I added the audio android-ffmeg library project in my application. I added a file called libsox.so in the libs folder of the application, it is called via a ShellCallback , looking in the file system. The problem is that it return a nullPointerException, depending on the mobile phone. On my Samsung it works without problem, but this problem appear on other device. The problem looks to be in the first line: fileBinDir = new File(_context.getFilesDir().getParentFile(),"lib"); soxBin = new File(fileBinDir,"libsox.so").getCanonicalPath();

Somebody know why it can't be found on some device ?

Thanks you