I had a look into the kernel source code provided by Sony. The good thing is, it looks like it contains the complete source code for the CXD3778GF s-master chip. So in theory modifications might be possible. It exposes several gain values to set and volume knobs internally. The interessting thing now would be, if the bootloader is unlockable. The source code of the bootloader itself is also opensource, it uses u-boot. However, I did not receive my nw-a105 yet, so I can not poke into it deeper.
If the bootloader is unlockable, I am very certain, that volume limitations of EU models can be removed.