I recently just finished scanning (converting) all of my parents 8mm movies to HD digital files. 34 files, 16.8 GB of new files, all with no sound, as there was not audio available for consumers in 1950s ans 1960s, or even until the 1980s.
One of the reels had this little ad from Kodak about how to add sound to your 8mm movies after the fact. The little flyer has a date on the back side of April 1961. Using an inflation calculator it is the equivalent of $3,425.73 in today's money.
I can't imagine many people paid that much just to hear sound. And it never mentions what having the magnetic stripe added would cost.
Now days, whip out the average cell phone and shoot broadcast quality HD TV in almost any light and get wonderful video with sound. Motion stabilized, and astonishing color clarity, and it does not cost a penny. Then watch them on a 60+ inch TV at home. My how times have changed in 62 years.