Are there any direct flights between Brunei and Mulu or I have to fly via Miri or some other city.
cheers
Yes, you need to go via Miri, Kuching or Kota Kinabalu. Miri will be cheapest.
Jadamros is correct. There are no flights from BSB to Mulu
Get a bus from BSB to Miri - then either stay overnight or go straight out to Miri airport. It will depend on the time of your flight to Mulu.
This post may help you:
https://www.tripadvisor.co.uk/ShowTopic-g303998-i9414-k13107786-Miri_to_Brunei-Miri_Miri_District_Sarawak.html