If you want to start thinking like a scientist right off the bat, I would suggest you start sketching these things out in writing:
1) what do you think you will find? How can you express it as a refutable hypothesis? For example:
"Different levels of compression affect the entire frequency spectrum equally"
2) A good scientist relies both on intuition and pilot experiments, and based on that decides on the best metric(s) to address the hypothesis. For example, before you go any further with your project, get a high quality uncompressed audio file from somewhere, and create three compressed versions: one only somewhat compressed, one compressed to the average or typical degree found in mp3 files, and one drastically compressed far more than is common. Look at the audio data with some tool like audacity that shows you the waveforms. Look at the video as a whole and try to get a tool that can display different frequency ranges. Try to see by eye what the most striking changes are from compression as compared to the original uncompressed data.
3) Figure out some simple numerical metrics that can represent the changes you saw by eye, and make charts of them where the points on the graph represent the original file and the several compression levels. It might be that the effects are different for different frequency ranges. It may be that you can get interesting differences simply by measuring the average amplitude in each frequency range (plus the standard deviation). Maybe the average frequency in each range says something, possibly because certain frequencies get degraded or exaggerated. Maybe it's the number of peaks above a certain amplitude level.
I have no idea what to expect, but this is how I would start getting a handle on the problem. Once you have these preliminary results, repeat the experiment with more levels of compression so that your charts have more points that trace out smoother curves that show your conclusions in a more convincing and aesthetically pleasing way. People should be able to read your brief conclusions, look at the charts, and say "yeah, I see that." It should take them only a few moments to understand what you did.
It may sound like cheating, but scientists don't coax information from Mother Nature without first poking and prodding her, and gradually exploring what she's doing. I suggest you do the same. If you have access to more sophisticated ways to analyze the frequency ranges, by all means use them.
Good luck, and let us know how things go!
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