We saw a group of hoolock gibbons in Hoollongapar(?) reserve forest near Kaziranga National Park in Assam. This was in March 2008. We went by foot accompanied by a forest guard with a rifle.
As we walked through the dense evergreen forest we could hear the hoo-hu hoo-hu calls loud and clear.
We reached a clearing and high above us among the trees
we could see 5-6 gibbons swinging on their arms and moving from tree to tree. It was a sight to behold. There were full black gibbons and also brown ones.
We saw one with a young one too. We kept watching them for close to an hour and then trekked back.