Square Enix did a poor job when compressing the audio for the pre rendered Bink movie files on the PC version.

This mod adds in higher quality audio tracks taken from the PS3 version's pre rendered movie files and patches it into the PC version's pre rendered movie files, thereby improving the quality of the audio and makes them sound nice and clear.

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/finalfantasy13/mods/34