if it's expected behaviour then would it not be better to script beeps IN to what you want with beeping turned off in mirc. I don't think it IS a bug...
(computers do what you tell them to do, not what you want them to)
you set the default behaviour in your mirc and haltdef is a text command, not a sound command. how about
on 1:invite:#:{
var %i $vol(wave)
vol -w 0
timervol 1 3 vol -w %i
}