One of my Whatsapp voice notes saved is not starting with PTT; what does it mean Whether whatsapp voice message be saved other than name starting with PTT; if so what does it mean...why its not stored with the mobile number details along with it; I deleted one of my contact; I have a voice note; but its not starting with PTT; its a alphanumeric number; and I could not find the mobile number associated..
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.

I noticed my voice recorder names all recorded audios with in a number format to confirm this. Hence its still recognised snd plays well on your device. Perhaps they sent you an audio created by a native voice recorder not via WhatsApp.