[YouTube-API] Is legal hide image of video and play only audio?

Hello, I would like to know whether it is legal to hide the image of the video as it plays to hear only the sound.

 In terms of youtube I think that he says it is not legal. But I find sites that do it and let me know if it is possible to do so as I see that they make sites like http://www.freemake.com/free_music_box/ (which allows hide videos on youtube while they reproduce and cover advertising) or while listening to the site http://fizy.com/#s/1ajra6 that allows to hide the video.

 I would like to know if it is legal to do so to apply to my web project. Thank you.

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