I was just wondering in K2 if we can add a Video or Image or Sound label to an item to notify visitors on the homepage that this article contains some video, image or sound.

See the site www.7sur7.be as example

and attached images

Of course you can do this. You just have to modify the appropriate listing or K2 content module within their overrides, read here on how overrides work getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates
and include a check for the above needs, video, image or gallery. You cannot have different notification for sound files since both video and sound are loaded through the same K2 section, Media.

You should advice your template provider/developer in order to help you achieve this.

Cant it be done manually like for image, where i can choose which icon to tag an article? can you help me with this?

No, I'm afraid this cannot be made manually. It requires some coding before you can just add the icon you want for each case.