add Programmyday form github

AudioPlayerMusterlösung/MP3File.cs

@@ -0,0 +1,17 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace AudioPlayerMusterlösung
+{
+    internal class MP3File : AudioFile
+    {
+        public MP3File(string filename) : base(filename) {  }
+        public override void Play()
+        {
+            Console.WriteLine($"MP3: {GetFileName()}");
+        }
+    }
+}