You should create a for loop, test your extension, and then add it or not...
Something like;
Dim AllowedExtension As String = "mp3 mp4"
For Each file As String In IO.Directory.GetFiles("c:\", "*.*")
If AllowedExtension.Contains(IO.Path.GetExtension(file).ToLower) Then
listbox1.items.add(file)
End If
Next
Or even more dirty;
IO.Directory.GetFiles(path, "*.mp*")
Or do it twice;
add
ListBox1.Items.AddRange(IO.Directory.GetFiles(path, "*.mp3*"))
and
ListBox1.Items.AddRange(IO.Directory.GetFiles(path, "*.mp4*"))