1) do you mean have it cut the sound effect and create the a new file or just store the sound in a audio channel?
2)This is rather easy to do. I have written a voice recognition program so I know how to use a mic to record. Though This would be more for development and when it comes down to it is not a feature you would use in a games development. The audio manager is for in game sounds and playing files. Plugging in a Guitar and playing a sound while playing a game would be rather difficult. And alot of APIs have line in options and drivers so it can access any line in via a adapter to the audio card.
3)the audio engine can already play back as many files at once as you can store in ram. This is not new at all it's a basic feature. Think background music and special effects music ther all just sound files in the end and play at the same time.