Speech is produced by the passage of air through the phonatory organs. You can refer to these Wikipedia articles about phonatory process (production of speech) and the phonatory organs.
Here is an article by a linguist "Pronunciation is a physical exercise" giving advice to people learning a foreign language, ending with:
The conclusion is that you need to know how to position your lips, teeth and tongue, and how to control your breath while saying a sound.
See also his other more technical articles: Speech and the Respiratory System, The Speech Organs and Airstream, Sound (Related to Speech). He also answered a question on EL about the pronunciation of the English "th".
So, obviously yes, watching someone speak is essential when learning the sounds of a foreign language. When dealing with beginners language teachers often ask their students to watch their facial movements (placement of lips, tongue, etc.) as they produce a sound, and this is important when the sounds of L2 do not exist in L1.
Filming can also be used to correct pronunciation. Here you'll find a research paper: Articulatory Training on Facial Movements Using the Webcam Pronunciation Mirror: A Pilot Study reporting on "a pilot research experiment to determine whether incorporating video and auditory feedback of the learners’ own productions into pronunciation lessons would serve as a viable mode of instruction that effectively enhances pronunciation of these sounds."
On the website of the university of Iowa they have a tutorial associating diagram, description, sound and video, to help pronounce the sounds of American English, Spanish and German. The website is not updated but you can download the app on a smartphone.