The original A20 Bluetooth headsets had support for the HSP and HFP Bluetooth profiles. These are used for communication, like WhatsApp voice calls, but most Bluetooth devices will not use them to play media files, like a WhatsApp voice note or music. This will be why you are unable to play your voice notes through your headset.
If media playback is something you would like to be able to do with your headset, the A20 with Bluetooth V2 added support for the A2DP pairing profile which allows for streaming of media files as well as taking calls so they would allow you to do this.
I hope this helps!
Feel more. Do more. Be more. New to the community? Check out the Getting Started board. Help others find answers to your questions by clicking on 'Accept as Solution' at the bottom of a reply. If you see a post you like, make sure to give it a thumbs up!