If you can not download an audio file, video or a photo a friend sent you through WhatsApp, or if you fail to submit load one, first check that your smartphone is connected to the Internet.
If you're using a WiFi network, it can sometimes happen that the router signal is strong and clear, but the network is temporarily out of service. In such cases, although it seems to have coverage, you are actually offline. Therefore, make sure everything is working properly.
After this, make sure you have free space on the SD card. Go to Settings> Memory, and there you will see what is the amount of memory you have and what is still available.
If there is little, you should release a pike. A good idea is, for example, delete photos and videos and save them online.
Another possibility is that, for some reason, can not write to the SD card (perhaps because you are in read only mode: try to copy files and see if it lets you do).
If space but the files can not be copied, drag all you have on the card to the computer (so you will not lose anything), then formatéala. , Put, if you want, the files in the SD and see that it is now possible to download files from WhatsApp.