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There is a well known issue related to this that gets replaced under warranty. Take to the Apple store they have a way to test it.

A quiet microphone or speaker, on a phone or earbuds is usually due to grease or wax and it may not be visually obvious. It's not a pleasant fact, and it's nothing personal it happens to my gear too. There are various ways to clean them depending on the device, google should be able to help.

I had a similar problem with an Airpod microphone (only on one side) starting to get quieter with intermittent drop-outs. It was likely a hardware problem and Apple exchanged it for a new pair after examination.