Download files like mp3, videos and application.

Good day!
I have a question about download, how can I know if the files like mp3, videos and applications that I download at many sites are safe and doesn't contain viruses, because sometimes there are incident that the anti virus that was installed on my computer doesn't detect the virus, although that anti virus was always updated.
Many sites are offering free downloads. Can you please give me tips to know, if the files that I am going to download is safe or not?
