This problem in Windows is not new to version 8.
I suggest to create a new power plan in Control Panel -> Hardware and Sound -> Power Options -> Create power plan, call it for example HDMI Audio, and set the new power plan to disallow the monitors sleeping.
Whenever you play audio, change the power plan to HDMI Audio and put it back to Balanced afterwards (or whatever is your normal power plan).
You could create desktop shortcuts to to do this with the click of a button, using Powercfg.exe.
First list all your power plans by name and GUID :
powercfg.exe -l
This will get you the GUID of HDMI Audio as well as of Balanced, so you can switch to any one of them using the command :
powercfg.exe -s GUID
You might need to define these shortcuts as to be Run as Administrator.
[EDIT]
There is a very long thread on this problem, starting in january 2010 :
Disable 'Monitor Off' detection, how?
It is worthwhile going through the entire thread, for there are some solutions mentioned for specific video cards.
The one solution that more than one person found to work, was disconnecting pin 19 of the HDMI cable (you might consider buying a cheap cable for testing) :