Just got some support from the vendor that sold me this camera and I've sorted my issue now.
So, for anyone experiencing a similar problem here's something else to check.
Within the video/audio configuration menu where you need to set the 'video type' to video&audio remember that you must do...
I have the same issue here with DS-2CD2443G0-IW however I enabled the options mentioned in this post (I followed this guide: ftp://ftp.hikvisioneurope.com/How%20to/How%20to%20set%20audio%20live%20view%20and%20playback.pdf) and I'm still having the same problem - "audio on failed" when I try to...