Nero - How to increase audio volume when burning a CD/DVD

I just burned a few CDs from APEs. While some of them came with a good audio volume (actually all from the same artist / editor), 2 others have e very low audio level, too weak to be listened to without raising a lot the volume.
How can I increase the volume of these 2 compilations before burning in Nero (v.7), and how to have them with the same audio level as other compilations I just burned, for example?