Although I see film as a visual art-form, there is no doubt that sound plays an important part; whether it is silent, ambient/non diegetic sounds, or musical soundtracks. My personal preferences hover between ambient/diegetic sound and silent. Brakhage raised a very important issue in that sound can impact your visual attention; watch a short with music being played on the soundtrack and then watch it silently. You will have different experiences and notice your visual attention is lessened with the soundtrack playing. Sound can also enhance or completely destroy the rhythm of a film.
In experimental cinema, there are only two artists who I believe truly understand the relationship between musical collaborations – Paul Clipson and Makino Takashi. They choose the musicians and the music almost feels like it is matched with the visual rhythms and often makes the film an experience of aural/visual combined. Sadly, most films with added soundtracks fail and for me often pollute the viewing experience. Jonas Mekas is a prime offender; his visual diary works are wonderful and when viewed silently are beautiful works and the rhythm feels more natural. Conversely, when watching Mekas’s films with sound on, his grating voice/narration which I feel adds nothing of value to his works, with the combination of horrible and completely out of place music make his works much much weaker. I feel his works should include the original ambient/diegetic sound (and when you see small passages of his works where he has included that and no music, you can really see why).
Films which I believe show the potential of sound/image combined are: Hypnosis Display / Another Void [Paul Clipson], Ghost OT 301 [Takashi Makino], Sleep has Her House [Scott Barley]. Those are some of the examples where non diegetic soundtracks have provided a multi sensory experience.
So, after viewing hundreds of films which I feel were polluted by unnecessary soundtracks, I’ve decided to index those films so I never view them again with the sound on!
Some interesting articles regarding sound and film.