Here's a couple of suggestions.

Use Audacity to cut your sound files. Just click and drag over the part you want to cut off and press ctrl + x or backspace, whatever works.

Sounds have to be 4.9 seconds or shorter, as Neko said.

I'm not entirely sure if this could work, but try buying a used kiai from the market and clicking edit to upload the sound. I just recently changed my kiai sound from my old one and it seems to have worked perfectly.

Lastly, wait a while in game. Sounds, like textures, may take a while to show up.