Au au is the written form of a dog bark in Portuguese and several other languages, similar to woof woof in English, which is why this short clip is understood even without context. Short animal sounds like this spread through soundboards and short form video because they are easy to drop into edits and instantly change the mood of a scene. The two bark pattern works as a standalone punchline, a pet voiceover, or a cute interruption, and its brevity keeps it popular for memes, family clips and pet content.