To use the Sound Manager with C# you just need to add a using NathanHoad;
statement to the file and then call PascalCase versions of the methods outlined in Sound effects and Music.
For example, to play a sound in GDScript you would call SoundManager.play_sound(some_sound)
but in C# it would look more like:
using NathanHoad;
And then...
SoundManager.PlaySound(someSound);