What you can do to figure out what name the enum value names is to give a random value to a variable of the type you want, the see the "Cannot convert bla into Godot.bla" error. The enum name will be that. In my case, I wanted to change the format of an AudioStreamSample. This has enum values. I made a new variable of type AudioStreamSample, the changed its Format to a random integer like so:
AudioStreamSample sample = (AudioStreamSample)ResourceLoader.Load("res://fart");
sample.Format = 3;
Next, an error ocurred:
Cannot convert type 'int' to 'Godot.AudioStreamSample.FormatEnum'
And there you have it. Just change the 3 with Godot.AudioStreamPlayer.FormatEnum.ImaAdpcm
(The last thing is whatever the value is in c# format, so capital letters to replace the underscores)