What happens when you add the file to the bottom of the playlist without loading it into a player at the same time? Is the duration displayed correctly then?
A little background: mAirList has two ways of determining the duration of a file: (1) By looking into the file and analyzing its header. This is works for MP3, OGG and WAV and is always performed when adding a file to the playlist (or the OTF database, which uses a playlist internally). (2) By opening the file with BASS and asking BASS. This is done when you finally load the item into a player.
Also remember that WAV is a container format which can contain audio data in many different flavors. The most popular is linear PCM, and mAirList should be able to handle these. What format do your WAV files use? Could you perhaps send me a short sample?