Most TV episodes from legal streaming sources are on Dolby Digital Plus. My reciever doesn't have HDMI and I don't want to listen to a 2.0 downmix.
How can I get the 5.1 audio without using a video converter (it would take its time on my old 2 core PC)? Is this possible? Any help would be appreciated.
What device? What exactly are you trying to watch?
Have a look at Kodi. It has sophisticated audio settings e.g. selecting an audio output (e.g. analog, HDMI, toslink), whether that connection can use passthrough 5.1, and if certain codecs aren't supported, transcode them on the fly if need be.
You' may need to set it up to transcode to multi channel PCM
The Mi Box has a Toslink/optical output. If you look at the 3.5mm jack you should see a telltale red light in there. You'll need an appropriate cable like this:
As far as I know Kodi can transcode DTS/DD+/DTS-MA etc. audio to DD on the fly for cases where you only have optical connected.