I have a 7970 that works without sound problems using SCCP.
Check on your firewall (Advanced -> Network -> Firewall rules) that you have a rule that allows incoming RTP (sound) packets:
IP version: both
Source port: 10000 to 20000
Thanks but it did not make a difference. Since I already was able to receive sound using the orbiter on the core/hybrid I am not sure if this made any difference in any case.
Anything else you can think of that might cause this? I am a bit confused since there is not much to configure. The cisco is pnp, the sip provider is straightforward and the core is directly connected to the internet. hmmmm.