First the MAS 6000 is a wonderful product and actually has a very powerful firewall. It was originally created by eSmith a Linux only company. Mitel has done well with this product.
As for using Microsoft SBS 2003 as a gateway/router, you can do this, as long as you have multiple IPs from your ISP. It sounds like you are trying to make the solution work as cheap as possible (or you would have multiple IPs). You want cheap, you have cheap... AND IT WORKS. AND, NOT ONLY DOES IT WORK, it is extremely easy to setup and maintain.
Getting back to the original posting/question, your PIX 515 (a good product as well) is not permitting the audio, because the ports used for audio and the ports used for call control (MiNet) are not the same. In your tele-worker manual you will find a list of all ports that need to be open. A solution you may want to consider is a Hardware VPN with the Crisco, pardon me, Cisco products. By doing this, your Mitel IP Phone will beable to talk directly on the same network as the 3300/200 without the need for the MAS 6000 box.
Let me know if you still continue to have problems with your setup....
MITEL... It's about | YOU!