Something

  //parsing CommaExts and passing each ext to WMPRegMultimediaExts
  if Pos(',', CommaExts) > 0 then
  begin
    S := 1;
    for I := 1 to Length(CommaExts) do
      if CommaExts[I] = ',' then
      begin
        RegExt := Copy(CommaExts, S, I - 1);
        S := I + 1;
        WMPRegMultimediaExts(RegExt, PerceivedType, MIMEType, ExtDesc);
      end
      else if I = Length(CommaExts) then
      begin
        RegExt := Copy(CommaExts, S, Length(CommaExts));
        WMPRegMultimediaExts(RegExt, PerceivedType, MIMEType, ExtDesc);
      end;
  end
  else
    WMPRegMultimediaExts(CommaExts, PerceivedType, MIMEType, ExtDesc);