I'm having similar issues. When sending whatsapp voice messages, they get "muted" randomly, at some point during the message recording. This has happened even shortly after a fresh install, so I doubt that clearing caches and re-installing can solve anything.
For what it's worth, I have also had the same issue while streaming live Facebook videos (interestingly enough, Facebook own whatsapp, so mabe there is some software shared between both apps, don't know...).
All in all, the phone is unusable for me. I have done several factory-resets, and tried the "phone repair" option in Xperia Companion, but nothing has helped.
Initially, I suspected it was due to a faulty phone, so I ordered a replacement and got a new phone. But the problem is still there... pfffff
