As eppie says you need to normalize the Wav. files. Digital programs are made up of Wav. files. They should be engineered to a standard. The common ones are AES in USA and EBU in Europe. However the db. difference is very small. Programs can normalize a whole batch of files at once quickly. There really should not be differences in the levels like you describe. This is because of ignorant boneheaded engineers not willing to adhere, or ignorant of agreed standards. So what you need is a Wav.file editing program, on a pc. I think there are some inexpensive ones out there now. Then you put the files into Wav.files on your PC and make a batch file to normalize and then transfer it back to your USB drive.