I want to work and mix in 5.1 mode but I can only map FL and FR speakers. I have set the proper audio hardware device (I have Creative Sound Blaster, SB Audigy). It's a strange and frustrating problem because I have Cool Edit 2.1 and there I work in 5.1 and everything is fine. Is it some kind of extra audio driver problem or what?
...which is important, because Cool Edit 2.1 uses the WDM / MME approach to get the 5.1 speaker outputs to work.
Now, because of all the clamoring that led up to it, we only have ASIO in AA CS5.5 and CS6 (you didn't say which version of Audition you're trying to run). That's great if the Audigy has a full-featured ASIO driver. If not, you might try ASIO4ALL, a free ASIO wrapper. If that doesn't work, you may have to invest in a pro-level sound card to achieve what you want.
On a Creative Labs Lies device, you almost certainly can't do this. Their drivers are minimal, and generally not very good. If the MME driver is only a stereo one (most likely) then you'd have to use ASIO - and their ASIO drivers are even worse. If it's that important to you, you'll have to get a proper sound device with a correctly implemented ASIO, or an enhanced MME driver. But essentially, what the sound device manufacturer's driver offers is what is shown up in Audition as available for use. I'm afraid that if it ain't there as a selectable option, you can't use it - even if it looks as though you can in a physical sense.
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