I am in the concept phase of an application that is going to have a lot of Audio/Video input and output. I want to do it in Java; but somehow am not fully convinced yet. What do you think? How bad could it be? And any advices?
Why I am thinking Java:
C++ would be the first choice because of performance concerns often present in Audio/Video processing as well as the range of available libraries for video/audio.
You do make a good point about being familiar with Java. If you are pressed for time, this is even more important. However if you can spare some time for learning, C++ would be well worth it.
Regarding .NET: The .NET SDK with everything you need is freely downloadable. Get hold of a free IDE such as SharpDevelop and you are up and running. It's Visual Studio that costs money, you don't need Visual Studio to do .NET development.