Goods manufactured in Thailand (most goods, audio components included) can be imported into the US duty free. America has a special agreement with about 140 countries to help them grow their export businesses, Thailand is one of those lucky countries.
The shipping companies (UPS/FedEx) usually know this but sometimes they mass clear many packages and pay duty on shipments from Thailand, even though it is not due. This is when they charge you, when they make that mistake. It's easier to charge you than to apply to get the Duty back. But... you can protest and get it back after some time and some hassle.
We ship from Thailand to the US all the time, and never have had to pay import duty. Some handling fees or taxes may be due but never Customs duties.