5X no sound through earpiece during phone calls

My wife's 5X was working normally yesterday but today cannot get any sound through earpiece during telephone calls. Speakerphone mode works fine and the other party can hear her regardless of speakerphone or normal mode - it is just the earpiece that is dead, and I would say "totally dead" as in no sound, no crackling or static, nothing at all.

She ran a road race today and the phone was in her waistpack with the face towards her body where it was presumably exposed to perspiration. Possibly related, possibly not.

Yes, I did reboot and yes, I did ensure that in-call volume was turned up. No, there are no active Bluetooth devices and I even disabled Bluetooth entirely.

Any other ideas? Unfortunately, the phone is 13 months old and thus, I assume, out of warranty.

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