xamarinmac 
namespace System.Speech.Synthesis
{
    public class PromptBuilder
    {
        public void AppendText(string textToSpeak, PromptVolume volume);
    }
}
.NET
5.06.07.08.09.010.0
.NET Core
2.02.12.23.03.1
.NET Framework
3.03.54.04.54.5.14.5.24.64.6.14.6.24.74.7.14.7.24.84.8.1
.NET Standard
2.02.1
Information specific to xamarinmac
Assembly

System.Speech , Version=4.0.0.0, PublicKeyToken=31bf3856ad364e35

Referencing

Your project needs a package reference to System.Speech.

Package System.Speech (10.0.0-preview.1.25080.5) netstandard2.0
Platform Restrictions

This API is only supported on the following platforms:

  • Windows
  • Built-in API
  • Package-provided API