In the iOS 12.2 beta, Apple has improved the quality of the audio messages sent in the Messages app after switching to a new Opus codec at 24000 Hz, up from the previously used AMR codec at 8000 Hz.
What you want is BarbaBatch but it's not free. It does handle the format you require. Might be a free trial. Audacity (freeware, google for it) will export to A.law encoded wav. Don't know whether it ...
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