Square Enix did a poor job with compressing the audio for the pre rendered cutscenes. The lower quality compression of the audio file is easily distinguishable especially on the music tracks that are playing at a higher volume than the other sounds in any of the pre rendered cutscenes.

This mod adds in higher quality audio from the PS3 version to the PC version's cutscenes and this should make the PC version cutscenes sound a lot better than before.

You can download the mod from this nexus page linked below and follow the instructions in the description page to install the mod.
https://www.nexusmods.com/lightningreturnsfinalfantasy13/mods/3