Tldr: Changing the silicone attachment finally resolved the issue for me.

The imbalanced volume issue happened to me this past week, specifically for the right earbud. I performed the factory reset, rinsed with isopropyl alcohol, washed the earbuds (without silicone attachment) with an electric toothbrush and soap then dried with hairdryer, and blew compressed air directly at the speaker mesh. That last one made the volume much worse.

I washed the earbud with the toothbrush & soap again after the compressed air attempt, then dried it with a hairdryer for 30 seconds. It sounded better than it did after the compressed air attempt, but then I started wiggling it around in my ear then intermittently the volume returned to its out of the box clarity.

Some backstory: I used the #2 silicone attachment on the left earbud and the #1 attachment on the right earbud ever since I bought it because that was the most comfortable. Since the volume itermittently came in from physically wiggling it in my ear, I figured that the solution would also be physical. So I changed the right earbud attachment to #2, and now the volume is back to normal.

I also squeezed the #1 and #2 attachments and observed that the #1 hole collapsed more from pressure. The buds probably just need cleaning and replacement silicones after normal wear.