I installed the OpenVPN daemon a couple of days ago so that I'm able to connect into my home network from anywhere and route my traffic via this encrypted connection - helpful for public or hotel WiFi.
Looking into the MediaTomb logs at /var/log/mediatomb.log told me that it used a different IP address - one from my VPN address space:
2015-06-30 23:03:08 INFO: Server bound to: 192.168.4.1
Since the netmasks of the radios are /24, they could not connect to the server anymore.
The .4.x addresses were on device tun0. I had to adjust /etc/default/mediatomb to force MediaTomb to use the correct network interface:
That's it. Radios play music again.