1. He likes watching loud videos.
2. He likes watching erotic videos.
[Can we use these expressions? Are both the same?]
3. The video is erotic.
4. The video is loud.
[Can we use these expressions? Are both the same?]
All the sentences are grammatically correct. There are differences between the two adjectives, though.
http://www.dictionary.com/browse/loud?s=t
http://www.dictionary.com/browse/erotic?s=t