Found on YouTube, a video that the original poster, Good Dog Films, purports to be the "oldest Thai boxing footage ever". It is possibly from Khrai Di Khrai Dai (None But the Brave), from the late 1920s or early '30s.
The original poster's description:
This is the rarest Muay Thai boxing footage around. I got it from the [National] Film Archive [in Bangkok]. I had to get it transferred from celluloid onto digital. I also included a trailer of Fight or Flight.
The 5:31 video features the three-minute clip of a Muay Thai match, split by a trailer for Fight or Flight, a documentary about a Westerner studying Muay Thai in Thailand. The fight clip switches to the documentary trailer after about two minutes, and then resumes after the trailer is complete.
The original poster does not give the name of the old film clip, but his description of it matches that of a piece of ephemera held by the National Film Archive. From Cinema of Thailand, in Wikipedia:
Seventeen films were made between 1927 and 1932, but only fragments have survived, such as a one-minute car chase from Chok Sorng Chan [Double Luck] or a two- to three-minute boxing match from Khrai Di Khrai Dai (None But the Brave).
The film fragment has a voice-over and a soundtrack of the traditional Muay Thai music. The tone of the voice-over commentary is similar to that of Merian Cooper's and Ernest B. Schoedsack's 1927 docudrama, Chang - that of a wide-eyed, naive Westerner describing something he thinks must be totally new and strange. It's a manner that says, "Gee, look at these quaint people." I wonder at what point the voice-over and music were added to the clip? Where, indeed, does it come from?
The clip itself bears the title "A Fistic Contest", and the following title card:
A much-beloved Siamese sport. They dance, kick and fight to the rhythmic strains of an orchestra.
The voiceover narration then kicks in:
The Siamese boxing contest opens with a sort of weird dance to the strains of an orchestra. The dance symbolizes that the boxers are looking over the ground for the combat. All during the round, the orchestra plays, starting slowly and getting faster and faster, encouraging the boxers to fight furiously.
Siamese boxing is the real art of self-defense. It teaches defense against everything. Kicks, fist blows, elbow jabs, knee punches or what-have-you. One of the favorite blows is knee to chin, which is often a knockout.
Here is another kind of blow, known as the rear attack.
The YouTube clip then switches to the trailer for Fight or Flight. At about 4:30, it resumes with the old footage. The voiceover narration continues:
Siamese boxing is not cluttered up with a lot of rules and red tape. They go in to fight and they do. Oh boy, what a fight! Look at that fellow, he's woozy! There he goes ... down! 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 ... by the count of 20 he is definitely out.