Sound for 'new message' on worldmap

[HaJo]

Can we have a sound for 'new message' on worldmap ?
At least for the important messages like new alien activity
and research gets finished.

Candidates are newmission.wav, popup.wav and roundstart.wav

Issue 1546340 on sf.net, see
https://sourceforge.net/tracker/index.php?func=detail&aid=1546340&group_id=157793&atid=805245