Event Sounds Question

I assign wav files to system events.
Certain ones won't play when when I assign it to the Windows Logon event.
It previews fine and plays in Windows Media Player
I thought the length was the problem 'cause these wav files are 22 and 24
seconds but I assigned a wav file to the Windows Logon that is 30 seconds
and it
plays fine. Although the length does have an impact.

Any ideas??
James