When I was on holiday in W. Sumatra, I stayed for a few days on the shores of Lake Meninjau. Even when walking in areas where there were no trees for miles around I could still hear the siamang. Their call is loud. There is a recording at: http://www.honoluluzoo.org/siamang.htm, about 2/3 of the way down the page.

Bingley

Bingley