You can compare decoded raw bitstreams with tools like Audacity but imho it is unnecessary. Re-encoding with VBR (Variable bitrate) encoding would be the way to go. It will use lower bits when there is less sound information, and will use your maximum provided bitrate when it actually needs it. Also, I would strongly advise to migrate from mp3 to a better format unless you're hardware locked. 128 kbps opus is said to be transparent (almost indistinguishable from uncompressed form to 99% ears) while 80-96 kbps opus is somewhat equivalent to 320 kbps mp3. And the format is royalty-free with wide support from various OSes.