Important June 14, 2015 Update: I just learned about a feature available in PT that can greatly simplify this process. Details are provided at http://duc.avid.com/showpost.php?p=2268770&postcount=26 
August 13, 2015 Update - The simplified process doesn't work on my son's PT12.1 - Dell Windows 8.1 laptop. This "old" procedure does however. http://duc.avid.com/showpost.php?p=2284049&postcount=3 has a link to an excellent video showing how to setup ASIO4ALL for use as a Pro Tools "Playback Engine". http://duc.avid.com/showthread.php?t=370379 explains how to set the PT's (and ASIO4ALL's) buffer size(s) to avoid sound dropouts during playback.
Pro Tools’ inability to work properly with Windows (ASIO) audio interfaces has been well documented on the DUC (i.e.
http://duc.avid.com/showpost.php?p=2264208&postcount=67 as well as many others). Although that thread was about “third party” interfaces, AVID’s own interfaces (at least it’s Fast Track Duo and Eleven Rack) have similar problems. My experience that other DAWs don’t exhibit these types of problems (except with AVID devices) strongly suggest that it’s a problem with Pro Tools, not the devices themselves.
Here’s the workaround that I developed to help mitigate some of these problems (when changing sample rates and interfaces) and help ensure that
Pro Tools 12 starts/re-starts reliably. (Many of the reported startup issues currently being reported by PT 12 users are likely caused by this bug).
This post doesn’t address the problems associated with changing buffer sizes - that’s for a future post (if there’s sufficient interest shown in this one).
1. Before starting Pro Tools
2. Go to
http://www.asio4all.com/
3. Download and install the latest version of ASIO4ALL (its Instruction Manual will appear in your “OS(C:) Program Files (X86), ASIOALL v2” folder) in case you ever need them (likely not now)
4. Turn off or unplug your audio interface (“Playback Engine”)
5. Start Pro Tools
6. Create a new session at whatever sample rate you want to use with your audio interface
7. Close Pro Tools
8. Turn on or re-connect you audio interface
9. Use your audio interface’s control panel to set it’s sample rate to whatever you selected in that Pro Tools session
10. Restart Pro Tools and open the session that you created with ASIO4ALL
11. Pro Tools will remember that you last used ASIO4ALL at that sample rate, so you’ll need to change it via “Playback Engine” in the “Setup” dropdown menu
12. You should be good to go with your audio interface until you need to change sample rates or interfaces (or PT decides to screw things up for you.) I’ve seen PT change interface sample rates while just closing and re-opening a session. I’ve also seen it think my third party devices were “Avid Hardware” and “HD Hardware”. Regardless of the cause you’ll need to repeat steps 4 – 11. If PT screwed it up, you should probably restart your computer first.
13. Note that ASIO4ALL (with your computer’s own non-HDMI audio device) will likely satisfy all of your non-recording needs and, unlike PT12’s new “Windows Audio Device” (and likely any other) “Playback Engine”, PT seems to be able to change its sample rate directly and correctly.
14. Good luck and please let me know how this works for you!