Silent Speech doesn't require vision of the target.
Silent Speech, unlike other forms of telepathy in 5e, doesn't require you to see the creature you wish to communicate with.
Silent Speech. You can speak telepathically to any creature within 30 feet of you. The creature understands you only if the two of you share a language. [...] (SCAG, 110)
Compare this, for example, to other types of telepathy in 5e, such as the warlock's Awakened Mind.
Starting at 1st level, your alien knowledge gives you the ability to touch the minds of other creatures. You can communicate telepathically with any creature you can see within 30 feet of you. [...] (PHB, 110)
If something requires vision or line of sight, it states so. Silent Speech doesn't, so you should be able to speak to a creature that you know is there, even if invisible or separated by a wall, using Silent Speech.