It's more likely that voice note wasn't created as as push to talk (PTT) message via WhatsApp in which case it normally starts with PTTxxxxx, rather this voice note was created with a device recorder hence starts with alphanumeric number.
It's more likely that voice note wasn't created as as push to talk (PTT) message via WhatsApp in which case it normally starts with PTTxxxxx, rather this voice note was created with a device recorder hence starts with alphanumeric number.