Everyone knows what the Tarzan yell sounds like in the version popularised by Johnny Weissmuller (and unlike most of his successors he actually makes it sound scary, especially if youre not expecting it):
But what did Edgar Rice Burroughs intend it to sound like? Thats much harder to say.
Burroughs simply described it as the supposedly terrifying victory cry of the bull ape. Unfortunately, there is no particular sound in real life that this description picks out: when apes are fighting, they mainly grunt or screech not in an especially terrifying way (and once a fight is over theyre usually quiet):
Theres one particular sound the word mangani or tarmangani (in Burroughs fictional language, meaning ape and white ape/white man respectively) yelled with a rising pitch that was used in the 1932 Tarzan radio serial and the 1935 movie New Adventures of Tarzan. Burroughs himself was heavily involved in both productions, leading some to speculate that this is Burroughs intended sound; you can hear it at 14:14 here:
But its not especially prepossessing.
If one wants a sound that seems like it might actually involve an ape, Id suggest something like this:
That one sounds like a cross between a lions roar and a gorillas grunt, which might be about right. But who knows?