No problem. 
Opened the .snt file I used and found these little properties:
<SOUNDENTITY>
  <SOUNDS>
      <Main>
        <Sound File="15_the_big_scream1.ogg" />
        <Sound File="15_the_big_scream2.ogg" />
        <Sound File="15_the_big_scream3.ogg" />
      </Main>
  </SOUNDS>
  <PROPERTIES Volume="0.5" MinDistance="20" MaxDistance="120" Random="0.5" Interval="60" FadeEnd="False" FadeStart="False" Stream="False" Loop="True" Use3D="True" Blockable="False" BlockVolumeMul="0.8" Priority="0" />
</SOUNDENTITY>
Could the bolded parts be the reason for the strange behavior? Random would explain why the sound wasn't always heard and loop + interval could explain why I heard the sound randomly.
If this is the case could someone tell how I can use custom .snt in my custom story?