Yes, there may be a problem with audio if you run the Vail SIP TIM behind a standard NAT firewall. NAT can take care of address translation at the IP layer. As a result, SIP INVITE requests get properly tranlated. However, the media IP address and port negotiation happens inside the SIP Envelope (SDP portion). The Vail SIP TIM will present its internal address for media (voice) transmission.

To resolve this problem, you should implement a SIP-aware firewall. Cisco makes one and there are others. A SIP-aware firewall typically can look inside SIP packets and perform the necessary address translation.