I know that a built-in sound player has been suggested before. I just wanted to make the same request. Playing sound files with Opus seems to be hit or miss. A sound player with more features including an image pane for the album cover would be appreciated. I saw that someone managed to make one work, Dopus Music Player but I couldn't find an explanation as to how it was done. The codec problem would need to be addressed in any case. Happy 
Go to Preferences / Viewer / Viewer Plugins, configure the ActiveX + Preview + Office + Web plugin and turn on the Windows Media Player preview handler (ignore the warning that appears at the top):
You'll then get the same player in the viewer pane which Windows Explorer has, which includes album art.
(The reason it isn't used by default is that you have to click to play things, while the default player plays things immediately but doesn't show album art.)
Thanks again Leo! I did as you instructed and downloaded codecs for WMP. Much better. 