The simplest method to make it work would be to use a media server product, such as Plex or Emby, or a DLNA server such as Serviio, rip your movies to the server and let the server transcode the audio into Dolby Digital. All three are available for free. Plex and Emby have dedicated channels for Roku devices, while DLNA servers use the Roku Media Player for access. The only downside (if you consider it as one) is that you need a device running the server software whenever you want to stream a video. For myself, that's no problem, as I leave my computers on 24/7 anyway.
Of course, you are referring to directly playing the movie disc from the BD player. In that scenario there's no way to get the DTS-MA track to your soundbar. Your TV will not process the audio. About all you can do is to configure the BD player to send two channel audio to the TV.
There's no way to extract the audio from the HDMI cable and preserve the DTS-HD track. At best it might convert the audio to 5.1 Dolby Digital, or maybe the lossy DTS core. At worst it converts the audio to analog stereo. For a bit of effort to set it up, you might find using a media server a far simpler way to go. Use MakeMKV to rip your discs to the computer hard drive, and then you can stream to all devices on your network. In most cases your Roku Streambar will pass the DTS core audio without transcoding, and if not the server can convert it to DD, so you still get 5.1 audio. Roku devices do support DTS audio over HDMI, but only from a MKV container.
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