Measuring sound levels from mp2,.wav files using a background program

I'm a 16 year old junior in school, attempting a project. I'm trying to find the sound( or relative decibel) level of a recording that I will record with my android phone. Since I cannot find the decibel level of the file without a reference point, I will have another recorded file who's decibel level is known, By comparing the two, I wish to find out the decibel level of the first recorded file. Can anyone tell me a software that could do this? I have a Macbook Air, and I have the software Audacity, but I do not know if it is possible to perform this particular task. I have searched all over the internet, but I have not found a specific answer.