Comparison of two waveforms of audio files

[Saurabh85]

Hi,
I am a VC++ developer and currently need help in comparing two audio files. Lets say I have two wave files and one of it is created using the other with some modifications like lowering the loudness etc.

Now, I have to compare these files and see if the modified one is almost copy of the original one, means, while creating the modified one, my application has unknowingly not distorted the file.

A percentage value would be better to signify how much different these files are. I have tried taking FFT of both files and then computing the difference in dB (like, 10 * log10 (ft1/ft2)) and averaging out the result. i get a resultant number but I am not sure what that number signifies.

Lets say I have a wave file and use it as an input to a software which decreases ,say volume, and gives output wave file. Now I want to compare these files to confirm that my software has not shifted the waveform or distorted it too much. I guess the shape of waveform must be identical, only amplitude would have been affected.

Thanks in advance for any kind of help.