Get the active form of folic acid. It is derived from plants, and is the proper dosage: 800 mcg daily is what is needed.
In this excellent peer-reviewed, randomized, placebo controlled hearing study from the Netherlands, 728 adults ages 50 – 70 were followed for 3 years.
At the end, those on folic acid heard low frequencies (deep sounds) better than those who did not take folic acid, which is the same as vitamin B9, and folate.
Modest amounts of folic acid can be obtained from food: green vegetables, all legumes, fortified cereal and in the grain supply in Canada and the U.S. Fewer neural-tube birth defects have resulted.
Most people need a supplement to get to 800mcg daily and prevention of hearing loss.
Supplementation with just 400 mcg daily is recommended to pregnant women to prevent birth defects.
If you take supplemental folic acid, you should also take B12, or make sure your B12 supplement is adequate.
These two B vitamins work together. The people in the research study had levels of at least 200 pmol per liter to start with, which is adequate.