I have yet to play the game, but from what I've seen and read I can't wait to play it!
The Gamautra feature explaining how the sound team at Radical went about creating and implementing the ambience in Prototype describes some useful interactive real-time mixing techniques and offers insight into the workings of their proprietary audio tool AudioBuilder.
One section that particularly interested me is how they use 18 channels of ambience simultaneously streaming from disc and dynamically mix the levels of individual tracks or groups depending on game variables - more specifically the density of pedestrians, infected hordes, and traffic building up around the player.
The system also uses these density values along with the position of the sounds relative to the player to dynamically mix the ambience among the quadrophonic channel set-up to help provide a sense of orientation.
This interactive mix of the individual ambience elements seems to be entirely reactionary to real-time variables. So, though I'm sure it provides an immersive ambience that feels alive, I can't help but think it would be nice to override this for events or moments in the game. For example, it could be effective to be able to duck all ambience apart from the infected hordes at certain points in the game to achieve a dramatic subjective type moment.
The overall ambience is bussed to the main mixer system and does allow for control of the ambience bus within the overall mix to apply filtering for cinematics and special game modes. However it would have been pretty cool to have the control to exclude a certain element of the ambience that is filtered, giving the ability to inject that extra bit of emotional delivery when neccessary.
Perhaps the question remains, would this be apporopriate to the gameplay or the character and story? I'm not exactly sure as I haven't played it but it would be interesting to see if that extra level of flexibilty in the mix could intensify the cinematic qualities in the gameplay and possibly even help provide more variation in the soundscape...
All things considered though it's great to see a complex and intelligent mix being implemented and also described so openly to the world!