This is a very simple mod that adjusts the Detection Sound Level of the Nightingale and Nord Hero bows so they're silent like all other bows in the game. A similar fix was included in the original version of the Unofficial Skyrim Patch and is probably included in the Unofficial Skyrim Special Edition Patch, but I made this for myself as a standalone and thought others might be interested in it as well.

Note, all five leveled player versions and the NPC version of the Nightingale Bow are fixed by this mod.