I need audio / song for my opening video.
Can somebody tell me where I can download that ?

Thx

How do you expect someone to answer this question? Picking
music to go with your video is a very personal thing, it should
support and enhance your movie. If you have a love type movie
it's probably not a good idea to put some death metal songs
with it.

Besides this you need to buy (pay royalties) the rights to use
a certain song with your movie. This can be quite expensive,
depending on the popularity of the song/group etc.

Lastly you can try to hook up with indepedent music creators
and you may be able to use their songs for free in exchange for
credits in your movie etc.

try www.freeplay.com or use your favorite search engine to search "royalty-free music" and you'll get a huge number of web sites peddling decent music with licenses included for a nominal fee. good luck.