I'm a bit late to the party but if you are looking now there's a nuget package available (AudioSwitcher.AudioApi.CoreAudio) that simplifies audio interactions. Install it then it’s as simple as:
CoreAudioDevice defaultPlaybackDevice = new CoreAudioController().DefaultPlaybackDevice;
Debug.WriteLine("Current Volume:" + defaultPlaybackDevice.Volume);
defaultPlaybackDevice.Volume = 80;
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