I went through the same thing. From what I can tell the AC3 codec is broken in the current build. So, if your video file uses this codec to compress it's sound, then Roku Media Player will have choppy sound or crackling. What's interesting is if you use "Play USB" then your file might work. I was surprised to find when encoding with hand break, I had put full audio tracks in both AC3 and AAC with some of my movies. "Roku Media Player" will prioritize the broken AC3, while "Play USB" will use the AAC. If you only have AC3 then "Play USB" will have silence instead of crackling.
To be able to play anything with an AC3 audio in Roku you need a media player that does not rely on Roku's programming for the codec, but has it's own. There are other players (with less features that I have not tried, like "Shelby Photo USB Player". Some with more that I hear work, but you have to be running a server program on a computer on your network (Plex, USB Media Box).
I personally am getting off Roku, I detest the company now for their lack of responsibility in this issue. I am going to use Kodi, which is a feature heavy media player. The problem with Kodi is that you have to use a amazon product or build your own using a Raspberry PI (which is what I am doing).
Otherwise best option is a smart tv, or a blu-ray player or a media player like the sell on ebay. I haven't tried these media players, because I was worried about user interface, but if all you want to do is play a couple of videos it is a good option.