Ok, I've been in that situation a while back when I was trying to unwind exactly what I did and didn't need for TOSlink/DD to be happy with PC audio.
You're right, Dolby Digital Live is the thing that takes a multi-channel audio stream at the computer (which a game can feed audio into without needing to know what the audio chip is going to do with it) and encodes it to Dolby Digital so it can be sent down the wire and decoded back to 5.1 at the other end. Dolby don't use that terminology any more (at least last time I purchased a motherboard and needed an optical for my AV Receiver they were not using the DDL branding at all) as DDL is now part of Dolby Home Theatre. Quick side-note: DTS have a similar technology and I think they still sell it as DTS Connect but I've not seen it in cheaper products like the chip on your motherboard.
The reason why movies and games are not the same is all about what you're asking it to do and how the data has been formatted. When you put in a movie (bluray, DVD, a not insignificant number of cutscenes in mainly console titles) then there is an existing DD 5.1 stream there. Your audio card doesn't need to know anything about it beyond "here is a stream of 0s and 1s, pump them down that pipe you have labelled TOSlink". It is simply a dumb forwarder of this already DD formatted audio stream (and at the other end it gets decoded and you have 6 channel audio). Obviously when you are actually playing a game there is no pre-mixed DD audio because it is generating the audio by taking mono sources and positioning them in space based on where the camera points and working out what it sounds like at each angle for the channels. So the audio card (if the game knows to feed it surround audio - which may autodetect, may look at what you tell Windows the connection is, or may need to be set in the game settings) needs to take these 5 spacial channels (and maybe a bass channel or it strips out the low frequencies itself) and turn that into a DD signal to put down that TOSlink cable.
Dolby Digital is a compression technology because the TOSlink optical didn't have the bandwidth for uncompressed, 6 channel audio. So if your audio card can't do DDL and compress those 6 channels it gets from the game into something that'll fit down the digital cable then it is screwed (or needs to do analogue output over several wires) and you're going to get stereo (which will fit down the cable's bandwidth). Some RealTek audio chips do DDLive, some don't (generally older or cheaper are strong factors for being out of luck - I think Gigabyte have a history of buying the chips that do do DDL more than other manufacturers).
Ok, so that's the 'why' of needing specific stuff and all movies always work but games don't.
In Windows you normally go to 'Playback devices' and select the playback (output) port and hit 'configure' in the bottom left of that window to tell Windows how many channels a port has. Snag: RealTek (driver) digital outputs do not have this option so if a game asks Windows how many channels should it output then this can all go horribly wrong. You can't just do the configure wizard and tell Windows 5.1 and how far back your speakers are. You need to use the Realtek drivers. When you do properties on the digital output port here you can play test signals with the 'supported formats' tab. This is all pre-encoded stuff so it isn't testing DDL but that a known stream of 0s and 1s can be decoded at the other end (and it sounds like you've already sent a DD stream by playing a movie and the AV Receiver has turned on the light to say it sees 5.1 DD and decoded it). On the final tab, 'Advanced', there should be a Default Format drop-down. If your Realtek mobo audio supports it and the driver is installed and working right then at the bottom of that drop-down should be 'Dolby Digital Live (5.1 Surround)' and this is the DDL setting you crave for your 5.1 solution. Assuming your game avoids the snag I mention above or can be wrangled into ignoring it with a setting (maybe an ini file, Google is your friend for finding the geek who already read through all the files and figured it out).
But what if you looked up your motherboard advertising and think you have a DDL/DHT enabled chip but no dice? Now I'm working blind as my older PC than the one I'm on I had the Realtek drivers (worst website ever with download speeds - they throttle them close to dial-up so getting the driver via ASUS is a smart move for your sanity as otherwise you're looking at a 2 hour download of a 70MB file) with their control panel and I think there was an area where you set the digital and could tick a box or something to enable DDL. My current PC (making sure I got the above instructions right) uses Windows Update to grab the recent enough drivers for the Realtek audio (DDL worked just fine with the default Windows driver it selected so I left it, my main PC uses the HDMI on the graphics card for audio and at some point I'll get round to replacing the 8800GT in this box to one recent enough to have an onboard audio chip). So I have a vague memory of a tab for digital audio and enabling it, but as I say, the basic drivers without taht control panel seem to be just working for me so hopefully you are in the same boat and via the above instructions you could find the DDL default to enable.
For reference: although I may have a different chip, I think mine is the ALC889 (but maybe that was the old computer I'm thinking of), Windows is giving me a driver version of 6.0.1.6410 dated 07/07/2011 on this PC.
Best of luck, if that doesn't help then I hope it gives you a better idea of what is going on and stuff to start searching for to get it working in the future.
Edit: I completely missed that you'd given the model of mobo you had.
You have a Realtek ALC892 with DTS Surround Sensation according to ASUS. This could be a problem. If ASUS had paid Realtek for it then that chip supports the "optional Dolby PCEE program" which should have provided the Dolby Home Theatre support (which has DD Live) I mentioned above. The DTS Surround Sensation is one of those technologies used by soundbars that take a stereo set of speakers and make it kinda seem like you have surround (until you rotate your head and your ears realise it isn't real) so this is not going to do the job of even DTS Connect in encoding a DTS signal (which the above reply indicates your AV Receiver doesn't support for decoding (you can test this for yourself in Windows by following the instructions for the 'supported formats' tab I mention above).
I suspect if ASUS had paid the license fee then they would tell you on their specs page that your motherboard supported DDLive. I suspect this means you're stuck with stereo or surround from sources that are pre-compiled into a Dolby Digital feed (and so cannot include stuff where you have input that changes the audio, basically you're stuck with movies only).
You might be able to take this knowledge and buy a cheapo soundcard that definitely has paid the DDLive (DHT branded) license and has a TOSlink. I'm not sure how great the options are but buying a different motherboard so you get the one that paid the $0.50 licensing fee is rather extreme. When I first found out what DDLive was was when I realised my motherboard didn't support it, I then made sure to buy a series of motherboards where a must have was DDL/DHT, and now I have HDMI on my AV Receiver so the audio comes out of the graphics card and I can buy motherboards on other concerns.
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