You must have your game files extracted and the .exe file modified to load loose files instead of from the encrypted archives. Luckily, a tool has been made to do just that.

Download U3M here: https://www.nexusmods.com/darksouls3/mods/286

Extract wherever convenient and launch. If it doesn't automatically find your Dark Souls 3 directory, go find it (usually in X:/Program Files/Steam/steamapps/common/DARK SOULS III/Game/) and
click "Unpack". It'll take a while to extract everything, so just wait for a bit. When it's done, click "Patch" to modify the .exe so it loads the extracted files instead of from the archives during gameplay.

At this point it would be a good idea to make a backup copy of the "sound" folder, just in case you mess something up irreversibly and need to start over.

Place "fdp_main_dlc2.fsb" into the Game's "sound" folder. Alternatively, you can download DSSI and insert the sounds yourself if you want.

LightSaberRKSSWAOn.wav > s000662611
LightSaberRKSSWAActive.wav > s000662612
LightSaberRKSSWASwing1.wav > s000662613
LightSaberRKSSWASwing2.wav > s000662613b
LightSaberRKSSWASwing3.wav > s000662613c
LightSaberRKSSWASwing4.wav > s000662613d
LightSaberOff.wav > s000662633

Sounds used from: https://www.youtube.com/watch?v=ot0WXDQaI7E

A side-effect of this, is that the other Ringed Knight weapons use those sounds.
The Ringed Knight Paired Greatswords will sound like a lightsaber when you use the weapon art. The Ringed Knight Spear will sound like a lightsaber when you use the weapon art, and when you use certain attacks when the
weapon art self-buff is active (R2, rolling R1, jump attack).