Where are WhatsApp voice notes stored on an Android device? I have checked the /WhatsApp folder (and all the subfolders under it) but they are not there. I also tried the /Android folder but I am not able to find an entry for WhatsApp there.

I am using a Nexus 5, if that is important.
Your voice files are also stored in the whatsapp folder.

The path is: 


For voice notes:

WhatsApp/Media/WhatsApp Voice Notes/

The file names are starting with PTT. The files are stored here if the sender recorded the note via whatsapp.
For audio:

WhatsApp/Media/WhatsApp Audio/

The files are stored here if the sender recorded the note in an external application and shared it to whatsapp