Hi Sebastian
I think you try to modify the default rule created by Check Point that's why you cannot modify the source or the destination.
Here is an example of QOS policy.
under ToIP, rules "QOS ToIP entrant" and "QOS ToIP sortant" belongs to the "ToIP" Class of service.
It is a diffserv class (EF Flag), so any traffic matching one of these rules, will be tagged with EF flag.
The rule "default" is the automatically created by Check Point, you cannot change the "Any" values of its. The last rule belongs to the "Best Effort" Class of Service.
You just have to add your "Voice" class of service once and create then the rules you need.
Hope that it can unstuck your situation.
Tomorrow I'll be at the office, so I'll have more time if you need.
Cheers
Tan