Why does my audio keep reverting to stereo?

I have an ASUS P7P55D LE motherboard with onboard sound and I am running Windows 7 64-bit, and I am using the SPDIF output from my motherboard to my receiver. On my receiver I can see which audio channels are in use (i.e. stereo or 5.1)

In the audio interface properties I can test DTS Audio and Dolby Digital outputs and both work fine, but when I try to play a game with 5.1 sound (I've tried Left 4 Dead and Dragon Age Origins) it reverts to stereo. I was getting this behaviour with the default Microsoft drivers so I installed the latest from the ASUS website but there is no difference.

I notice that in the Advanced tab of the playback device Properties dialog (accessed by selecting the device in the Playback Devices window in Windows 7 and clicking Properties) the Default Format only allows me to choose from various 2 channel formats so maybe that is something to do with it?

How can I get 5.1 output all of the time?

edited 18 hours ago

asked Nov 29 at 14:32

GraemeF
107●6

75% accept rate

this sounds suspiciously more like a problem with the games than with your sound drivers. you're sure you've enabled 5.1 sound in the games' configuration? – ~quack Mar 14 at 17:23

You mention the Advanced tab, where is this tab? In the game? In Windows? Are you sure your game is outputting 5.1? – heavyd Mar 16 at 6:13

I'm sure it's not the games - I've never had this problem on other computers running Windows 7 (although 32-bit and with different audio devices). I've obviously checked them all for options to force stereo and found nothing. – GraemeF 18 hours ago

I've amended the question to clarify which Advanced tab I mean. – GraemeF 18 hours ago