using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        var input = "<emphasis level='strong'>Well done for completing this section!</emphasis>{nl}<audio src='https://storage.googleapis.com/smebanking/budgie.mp3'></audio>{nl}Let's keep up the good work.";

        var regex = new Regex("<[^>]*>");

        var matches = regex.Matches(input);

        foreach (Match match in matches)

        {

            input = input.Replace(match.Value, null);

        }

​

        Console.WriteLine(input);

    }

}