I'm not sure this is a device issue. It appears the Roku does not do any audio decoding, just pass through. It is the content providers which are not streaming those formats. The issue is probably like Netflix's DD+. When I have that enabled, I get no audio out of my TV (when my AVR is off and in passthrough mode), because my TV is old enough to not be able to convert DD+ to stereo, through it can do DD. I either have to turn 5.1 off through Netflix's app, or turn the receiver on to get sound. If you have a device (TV/receiver) that cannot decode these more advanced formats, you are not going to get any sound, and there is no way for the Roku to automatically detect what your downstream devices are capable of decoding. You'd need to tell the Roku during set up what you can and cannot decode, or tell the channel, then the content providers would need to have multiple streaming options available for all users. With HDMI passthrough, and the Roku not doing any decoding, I don't think this is a Roku hardware issue. Though it's possible they may have restrictions on what sort of content formats they officially allow, I don't believe there is anything technologically speaking that would stop the newer HD audio formats on current Rokus. I believe you can test that through a Plex server. If you can serve content with HD audio through your Roku and still get HD audio out, then you should be able to do it with web/channel content.

TL;DR (To the best of my knowledge) Roku doesn't support any audio formats, as it doesn't decode, but instead passes through. It supports those formats now. It is the content providers which are not giving you the content formats you want.