It doesn't have any sound on either one of my boxes. Not only that, but it doesn't have sound on two different computers running different versions of Windose and using Windows Media Player or Media Player Classic (with AAC codecs). If I look at the "details" using explorer, the Audio section lists a bit rate, but nothing for the number of channels or sample rate (which is odd). Additionally, if I use HandBrake to convert your video it gives all sorts of audio sync issues and produces something without much in the way of sound.
It does play with sound using VLC.
I hate to use Windows as a benchmark, but I guess if you can make it happy you'd have a better chance of things working.
I tried reencoding one of the files using aac instead of ac3. That one works on my N1100 and also plays in Windows. Could you try it on you roku? It is in the public Khan academy channel, under "other" - "ck12.org worked examples". It is the last episode, named "secret audio test".
If that one works on the N1000 I'll reencode everything with those options.