The National Archives has a vast collection of audiovisual materials relating to the Vietnam War and they are all in the public domain.
Type 'Vietnam' in the search box to get a list of all types of materials held by the Archives, and then limit the results by type. Select 'Refine by type of archival materials' on the left hand side of the screen, and then click on 'photographs', 'moving images' or 'sound recordings.' If the item is available online, and most are, you can download the desired item.
These may also have some photographs, but they are less useful than the source above:
MUSOPEN is a source of public domain music, mostly classical
The Internet Archive has music, audio, video – I typed in 'Vietnam War' and everything from interviews with vets and Walter Cronkite coming out against the Vietnam War was there. Some good stuff.
Creative Commons Search is not itself a search engine, but links to sites which do search for different types of materials. Type in a search term and then click below on a source, such as Youtube, to see what is available.