It depends on the type of audio in the file you are trying to play and what your TV can support. For instance, when I connect through an HDMI connection I can play my old avi files since they only have mp3 audio, however my mkv content does not work since the WD attempts to passthrough the ac3 or dts track to the TV, and my TV does not support this audio.
For this reason I just connected it to my TV with the HDMI & RCA connection, selected stereo, and now the WD will reencode the audio signal to go over my RCA connection, which is a signal that my TV supports.
EDIT: Didn’t catch the second post in time, however it is possible that the other box was reencoding the audio. This is just my best guess since you had said the WD worked on a different TV, and from a quick google search it seems like the Emtec may not have had audio passthrough options, so it would likely have changed the audio to another format which the TV could support. There are a lot of variables here though so could be wrong.