Hmm... I have 3 VoIP devices: VOIPo ATA, IP phone from work (that is administered by our network administrators), and OBi device that I use for Google Voice and SIP provider for international calls. All behind a router with OpenWRT. So far, I never had a need to forward SIP ports for any of these devices. I know that at least other SIP provider is using 5060... on the second thought, I only use it for outgoing calls, so that's why there is no need for port forwarding.

Audio ports certainly work fine together... we have two simultaneous calls regularly, and never had any problems...