Resolve NAudio needing an app

[jskeet]

NAudio really does need an app target to get WaveIn working.

Ideally, only top-level projects should need an app target.

Idea, like IViewServices:

• Provide an IAudioServices interface, possibly in Model
• Create an implementation of that targeting net472 and netcoreapp3.1
• Inject the implementation wherever it's needed

[jskeet]

This was done a while ago.