1. I would like to hear custom sound for the notification message, how can I reach this?
Notification message of Desktop-Reminder is linked with the Default Beep of Windows sound system. Here you can customize your message sound.
2. How can I set my task every two weeks?
Desktop-Reminder supports only tasks with the interval of one week. actually you can only reschedule such a weekly task. To do that, mark the task as done in the week it is not needed. It will be rescheduled to the second week.
Similar solution can be used to repeat the task every two months or years.
3. How can I set my task every X months?
You can create two yearly tasks where the one is started at the beginning, and the other one in the middle of the year.