If you have acid reflux, the first thing to do is to stop its continuing damage and let the body heal; if not, the damage will become cancerous, and stomach cancer has a kill rate, if I recall correctly, about 90%. The pain is your body telling you of its high danger.
After stopping its continuing damage, the body will heal by itself. Proton inhibitors are effective at this, and proton inhibitors are relatively safe. The standard course for proton inhibitors is two to four weeks.
After the continuing damage is stopped, then work on how to reduce the acid through non-pharmaceutical means. A low fat diet means that food stays in the stomach for a shorter period of time, producing less acid. An inclined bed means that the acid doesn’t crawl up the esophagus when the body is horizontal. But none of these are assurred to be a cure, so you may have to return to proton inhibitors.
The reason acid reflux affects singing, is not primarily because of the stomach pain, but because the acid, mostly liquid and some acidic fumes, constantly weaken the esophageal muscles, and such muscles are critical to resonance. Also, such acids can affect the vocal cords– when this happens, you won’t have vocal tone control.
Most gastro literature state that the lower esophageal sphincter causes the acid to spill. The esophagus can tolerate a lower pH than the nasal-pharynx, so the other sphincter mentioned means that acid is spilling and is not damaging your esophagus nearly as much as the nasal-pharynx.
If you’re singing with a high acid stomach (condition of stomach or lots of food), and your technique is unusual, one can send more acidic fumes up to the nasal pharynx, which would cause loss of resonance control. This can be offset by drinking antacid before singing.
Remember that loss of muscle tone usually doesn’t occur immediately and healing occurs slowly. So, try proton inhibitors coupled with inclined bed, then the fat-restricted diet, then the spice restricted, then the quantity-restricted. Give each one about 4 weeks to work, and see what happens.
My approach is that the lower sphincter lost its elasticity due to continous poor posture– slouching, bad sitting (and even good sitting), computer work. If you can get your posture upright (difficult process), then the lower sphincter is automatically pulled away from the stomach, by a distance, I estimate, as long as one to two inches. This keeps the acid in the stomach, and reduces many of the other mentioned acid-reflux issues.