First Time PIM-SM: I am confused

Hi there!

I am currently trying to get Sonos Devices to work across different VLANS / Networks. According to what i figured out i need Multicast Routing for that to work.
I am trying to make use of PIM-SM for that reason.
My Sonos Speakers are in Subnet A, my Clients in Subnet B.

I created two PIM-SM Intefaces: Interface A and Interface B, each corresponding to the Subnet and the Interface my UTM has in those networks.
As RP-Router I chose the IP-Address of the UTMs Interface in Subnet A.

As Multicast Group Prefixes i chose 224.1.1.0/24 and 239.255.255.250/32.

It does not seem to work though. Maybe i did something fundamentally wrong here?

Can someone help me to understand this and configure it properly?

Thanks in advance!