I am sorry but i have to disagre. I have the same issue regarding Realtek ONBOARD Soundcard ,i have a MSI GE73 9RF Raider Notebook and it worked perfectly up until windows 7, on windows 10 started glitching now and then only Stereo option available in Audio Channels menu selection. I have tried everything from uninstalling,reinstalling,updating nothing works,watched a handfull of videos and read a LOT about this issue and it is a driver issue or Microsoft Windows issue and thats that.
- laptops usually have realtek alc2xx (stereo audio channel chipset) not alc6yy (5.1) or alc8zz (7.1).
- Your MSI GE73 9RF has spdif, but spdif only has enough bandwidth for uncompressed stereo audio.
The only way to get 5.1 out of spdif is by using proprietary audio compression formats like Dolby Digital or DTS.
For 17 out of the last 20 years, Microsoft has charged extra money for windows media edition so that you can have 5.1 dvd playback (and thus 5.1 dolby). Windows 7 (3 out of the last 20 years) is the exception in that Microsoft included a software dvd player for free without you paying extra money for 5.1 dvd playback. People get confused because 15 years ago, they bought a desktop pc with a dvd burner --- which comes with oem versions of windvd --- thus you paid for it indirectly. Or that you bought a soundblaster sound card --- which has dolby digital live or dts connect --- thus you paid for it indirectly.
So you end up with a lot of confused people because they had windows xp (with a dvd burner and windvd). Then they skipped Windows Vista (because everybody skipped vista). Then they have windows 7 --- which Microsoft gave away a software dvd player for free. Then people skipped windows 8 because everybody skipped windows 8. Then they have windows 10, which goes back to the norm when you have to pay extra money for 5.1.
If you have netflix windows app, your laptop will output 5.1 dolby digital plus (which is backward compatible to your av receiver) via spdif because netflix paid for dolby digital plus for their netflix windows app. Netflix specifically tells you that you can NOT get 5.1 by using your web browser because Microsoft didn't pay for it, your browser didn't pay for it and 99% of the time, your computer manufactuer didn't pay for it.

Take any realtek chipset such as alc898 and you will see that dolby is an optional item, which MSI has to pay for (MSI buys the realtek audio chipset and put it into their laptop, so naturally it's MSI that has to pay for it).

If MSI didn't pay for Dolby Digital for your laptop, then you will have stereo on spdif.