How can I retrieve the list of application audio sessions (like in Windows volume mixer) and change volume of one application (for example, Mozilla) manually (C++, Delphi)? Skype do something the same when you start talking (decreases the volume of other applications). Thanks in advance!
To enumerate the Audio sessions you must use the IAudioSessionManager2 interface and call the GetSessionEnumerator method, this will return a IAudioSessionEnumerator interface , from here you must invoke the GetSession method to return a IAudioSessionControl interface wich provides all the functions to retrieve the info related to the audio sessions. as additional lecture try the this MSDN article Default Ducking Experience.
来源：https://stackoverflow.com/questions/11665176/managing-apps-volume-in-windows-7