We have experienced similar problem. We are still chasing... We are only experienceing one way audio on outbound calls via SIP. We have a PRi and SIP trunks and our normal routing is inbound via PRI and outbound via SIP. We are running rel9.0.2.18 on a 3300 mxe. Mitel keeps pointing us to the number of SIP licenses we have. We have confirmed that the number of SIP trunks match from our provider and what is set on the 3300. To complicated things even more, we are currently running an OCS voice pilot thru the 3300. Out OCS server uses SIP to connect to the 3300 and send voice calls out.
On all the wire shark captures I have done, there is no RTP traffic after the intial call setup. Mitel has not been able to tell me why the 3300 is not sending it. At this point we are were asked by Mitel to capture both on the inbound and outbound side to see if the traffic patterns match? I have not done this yet. Right now most of our outbound traffic is going out our PRI as well, but I have a few phones (mine and my other engineers) going out SIP for troubleshooting and such.
I am not sure this helps, but if anyone wants for specifics about this problem please let me know. I will post more info if our troubleshooting reviels anything.