We have done a couple, it all depends on what you use and where you use it. I did it with an Axis P33 series camera, and tied it to a Larou Microphone / speaker kit. Worked well for the application. But audio is a tricky thing, if the mic is too close to the speaker you get feedback, if the mic is too far from the subjet, it gets too quiet. I really think you should give it a test before you commit to it. But you also need to make sure your VMS is supportive of the function you want. Also pay attention to local laws as if you want the camera to work as an intercom that is one thing, but if you install the camera speaker in a room and you want to communicate with all that are below, that is a different animal all together.
What is your application and the end result you are looking to do?
i am dealing with an end user whom I have proposed Axis + Louroe option for several of their indoor office area locations for two way communication, arguing on a competition offer for Geovision IP domes provides two way communication without external microphone & speaker.
For some basic 2-way communication using IP cameras can be a great solution and agree that an external mic should be used. But there is a need to mitigate customer expectations. This is not an alternative to a professional teleconferencing solution. Also, if used through a VMS interface, synchonization can be a big problem depending on what VMS and what camera are being used.