If it's getting cut off by itself (it starts playing again and that cuts it off), you probably have the Retrigger Percentage set to something below 1. Put that all the way up to 1. Otherwise I have no idea and you'd need to add more details.
I have a similar thing going on. Single quick 'tic' sound on a paddle prefab (2D Collision Enter) that hits a ball. Sometimes it sounds higher pitched and plays faster than it should, other times it's more dull but it's completely random and unpredictable. It's a single voice as the paddle can only ever be hit once within it's play time and all other settings are default. I did try increasing the number of voices but it still happens.
The ball prefab has 2x sounds on it, one for when it hits the dead zone and resets, and the other for when it hits a brick object and destroys it. The deadzone is a single voice, Brick is 10 voices in case it hits a bunch in quick succession but no other triggers when it hits the paddle.
It's rare, but happens maybe 1-2 times per level which can play the sound from 105 times in the first level and up to 175 times in fifth level.