5.8-1 Is it possible to add subtitles to a video of a song and upload it on youtube in order to facilitate the perception of this work to deaf people?

No, it is not possible to do so. Art. 24c CopA allows to to make a modified copy of a protected work of art, in a form which is accessible to people with disabilities. Such copies of the work may only be produced and placed on the market for non-commercial purposes, and only for the use of people with disabilities. In this case the copy is made accessible to everyone on youtube. Sothis reproduction is not allowed.