Amazon have just announced the Alexa Sound Library

To use these sounds Amazon tell you to add an SSML Audio tag. I submitted SSML support as a PR to Alexa.NET a while back, so to use these new sounds just use the following code after adding a package reference to Alexa.NET on NuGet

public SkillResponse FunctionHandler(SkillRequest input, ILambdaContext context)
  var ssml = new Speech();
  ssml.Elements.Add(new Audio(""));
  var response = ResponseBuilder.Tell(ssml);
  return response;