Nagios sounds

Hi All.
We are using Nagios to monitor our network. It all is working very well. Now I want to use a sound when, for example, a server is down.
I already tested is and it seems Nagios is only playing a sound when everything is normal.

host_unreachable_sound=hostdown.wav
host_down_sound=hostdown.wav
service_critical_sound=critical.wav
service_warning_sound=warning.wav
service_unknown_sound=warning.wav
normal_sound=noproblem.wav

It does not sound anything when a service is in critical state, or when a host is down or unreachable.
Does anyone know why this is not working? Do I still need to activate something else? :?:

Thx in advance for you answers!
8)
Arno