are you connecting to a digital out - ie SPDIF/TOSLINK, optical or coax? If the source is multichannel encoded with DD or DTS then the chipset should just pass this straight through to the digital out (as well as decode it for multichannel analogue outs).
In alsamixer, can you scroll over to the right and see a IEC958 option? You may need to enable it - actually "unmute" it by pressing 'M' on the keyboard. That should enable the digital stream out. Don't know anything about alsamixer, but I assume that it detects if there is an IEC958 available and only presents it if so. So if you can see it and it is "muted" then that is probably the issue. If it isn't there, then maybe it hasn't detected it? If it is there but unmute does nothing, then I've no idea.....
Col.