Currently, version 22.4.75 of WhatsApp for iOS is being distributed to users. This version brings a new feature. This is the possibility of continuing to listen to voice messages even after leaving the chat with the sender.
So far, you could not switch to other areas of the app when listening to a voice message in WhatsApp. If you exited the original chat with the sender, the playback stopped. After the update you will still have the voicemail player bar when exiting a chat.