Your text "AC3-/AC-3" is a tad puzzling. That dash after the first 3 doesn't seem like it should be there. I have MediaInfo in text display mode, and this is what an AC3/Dolby Digital track from a DVD rip displays:
ID : 2
Format : AC-3
Format/Info : Audio Coding 3
Commercial name : Dolby Digital
Codec ID : A_AC3
Duration : 2 h 23 min
Bit rate mode : Constant
Bit rate : 448 kb/s
Channel(s) : 6 channels
Channel layout : L R C LFE Ls Rs
Sampling rate : 48.0 kHz
Frame rate : 31.250 FPS (1536 SPF)
Compression mode : Lossy
Stream size : 459 MiB (7%)
Title : 3/2+1
Language : English
Service kind : Complete Main
Default : Yes
Forced : No
Now, your TV should certainly support standard AC3/DD audio, as that is the default audio codec used for OTA TV broadcasting in North America. You didn't mention what make/model TV you have, so I can't look up what it supports. But sometimes there are internal audio settings in the TV that you can change for different codecs. Sometimes some experimenting is necessary. Manufacturers don't always make it easy to use something other than the device they made.
Also, you didn't mention what model Roku you are using. Different models have different audio output settings. Yours is likely set on Auto, so you could try changing it to Stereo, which should force the Roku to output the audio as 2 channel PCM, which virtually every TV will support.
This is why I use an AVR on my two large screen TVs. Let the AVR handle all the audio and all the TV has to handle is the video. 😄
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