I think it's as simple as replacing:
if (Path.GetExtension(filename) == ".mp3") {
listBox1.Items.Add(filename);
}
else ...
with:
string ext = Path.GetExtension(filename);
if ((ext == ".mp3") || (ext == ".wma") || (ext == ".wmv")) {
listBox1.Items.Add(filename);
}
else ...
/ravi