O WhatsApp is reportedly developing a feature that will be able to transcribe audio messages. That is, if you are sent an audio message that you cannot hear at the moment, WhatsApp will be able to present the content in the form of a normal text message.
Information about this feature was provided by WABetaInfo website, which indicates that the beta version of WhatsApp for iPhone introduces transcription functionality. Since it is a beta version, this functionality still has some limitations, namely in the number of languages.
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It is not yet known when WhatsApp plans to launch this feature, but given that it is already being tested in the beta version of the app, it is likely that we will not have to wait much longer.