How WhatsApp send audio files or video clips instantly?
I guess that's because whatsapp stores attachments on their database with its own checksum (every file can be represented by an alphanumeric sequence of characters called like this).
When you try to upload a file whose checksum is already on their database, they just use the uploaded copy of the file. 
This leads to several advantages. Indeed, preventing double uploads:


Avoids server overloads
Makes you save mobile data
Make whatsapp save space on their database