Ok, I assume you have tried one of the sound files you use on the others (one that your 100% sure works)?
If you have already done the above there is only 3 things I can suggest which I'de do in the following order
1, deselect ALL other alerts and leave just this placement one with a sound file you know to work
2, log in using the 'factory default' profiles and try again
3, download and re-install the software again
Let me know if any of those help