My Beats Audio icon is no longer being displayed during playback
I got a reply from HTC technical support. Make of it what you will, I've asked the lady to clarify what she means as I don't understand her Engrish.


  Thank you for providing me with the additional details to enable me to
  resolve your enquiry. It’s always our aim to ensure that our
  customers’ issues are resolved first time and I’m sorry that this has
  not happened in your case. Thank you for performing the steps outlined
  in my previous email. I can confirm this has been removed in the 1.28
  update for your device, Headset setting they have now are gone and
  turns into “Other”. I am sorry for any inconvenience this may have
  caused you. I look forward to hearing that this has resolved your
  enquiry, to close your email or alternatively to reply to me please
  follow the links below. You will shortly receive an email asking you
  to submit your feedback, I would appreciate if you could take the time
  to let me know about the level of service I have provided you with.


Apologies for the formatting, the HTC support portal strips everything! :facepalm:

So, in order to see the icon again. 


You need to plugin a headset/headphones
Drag down your notifications bar, and hit 'Settings'
Click on 'Sound'
You'll notice the 'Sound enhancer' menu is now not greyed out
Tap this and you'll then get a choice of 'Sound enhancers' to use


Confirmed as working for me.