I contacted Whatsapp about this. Their answer has not really been helpful:
Here is what I wrote:
Hello
The WhatsApp audio messages don’t play over Bluetooth. You just hear a small amount of garbled noise or simply don’t hear anything.
I am on an iPhone 5, iOS 10.0.2, WhatsApp 2.16.11. I have tested with my Bose QC35 bluetooth headphones and my Plantronics Voyager Legend bluetooth headset.
No other app is showing problems. I have tested with Skype, Viber and Facebook Messenger and everything is working fine there.
The problems are only with the voice messages recorded with the device microphone and sent by a person, videos and other type of audio files sent via Whatsapp play without any issue.
Reading on the internet I have found several people reporting these kind of problems:
http://forums.imore.com/ask-question/338177-my-whatsapp-voice-msgs-doesnot-work- my-bluetooth-headset.html
https://origin-discussions-us.apple.com/thread/7677209?start=0&tstart=0
I hope you can come back to me with an answer about this.
Thank you.
This was their answer:
Hi,
Sorry for the delay! We have received many emails recently, and we do our best to answer them all. Thank you for your patience.
WhatsApp provides limited support for Bluetooth devices. You should be able to receive WhatsApp notifications and voice calls. Any issues you have are likely caused by the OS and not WhatsApp itself. Please contact your device manufacturer for additional assistance.
Thank you for your understanding and cooperation.
Sincerely,
xxxxxxxxxxx
WhatsApp Support Team
As WhatsApp is the only app struggling with this issue I believe it is not iOS what is in fault here, but their own app. As I wrote before Skype, Facebook Messenger and other apps are working just fine when playing voice messages over a Bluetooth device.
What is really strange is that the Whatsapp voice messages (human messages as other audio messages sent via Whatsapp do not show any problems) appear to be using a different audio codec than the used by the whatsapp videos. When you start to play back an audio message you hear a distinctive tone over the Bluetooth headset, this does not happen with the videos, where you don't get any tone.
I looks like we have to accept that this will not be corrected. Some of the reports date some years back so I don't think Whatsapp has given them too much attention. I just hope that this disappears when I upgrade to the iPhone 7.