You should be able to use
/splay $qt($wavdirmosquito.wav)
When using just mosquito.wav mIRC looks in the directory returned by //echo -a $mircdir
However, $sound is 'smart' enough to know to look in the sound directory(ies) as well.
mIRC sets 3 identifiers to reference sound directories, and these can be changed.
The identifiers are $mp3dir, $wavdir and $middir
By default these point to the same place, namely the sound directory off of your mIRC installation.