Minecraft wiki entry for Creating_a_resource_pack, associated text: The sound must be a .ogg file. Like in the previous section, you may use the free website Online-Convert.com or use Audacity as mentioned at the top of the page.
Also note the already existing files seen in your .json file from above. In this case, five other hurt sounds exist within the minecraft/sounds/damage/ folder, two for falling at different heights (fallbig.ogg and fallsmall.ogg) and the other three are when the player actually gets hurt (hit1.ogg, hit2.ogg, and hit3.ogg) Since we want to add a fourth sound to the existing three hurt sounds, we cannot reuse those file names. For simplicity, name your sound hit4.ogg.