How to turn up all channels at one time in mixer view???

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There are a few separate things to consider here... if you are happy with the overall balance then just bring the master fader down a bit. If you are mixing with the mixer's defaults (32-bit Floating Point) then you won't really be overloading anything except perhaps your sound device - you can mix down with the levels anywhere you like, and when you normalize the result to just less than 0dB, then all the clipping will cease. No it's not magic, but it seems a bit like it; Floating Point mixes are, in themselves, virtually impossible to overload.

So selecting all of the channels = bring down the master fader - that will stop the local overload. Your sound device gets Integer signals as it's a real-world device, and they will overload - but not the mix itself.

As far as the faders are concerned, Audition's mixer doesn't support fader ganging, but there is a better way to do it - use buses. This way you can control individual sections of your mix separately. So it you have all the drums going to a bus, you can adjust the balance of them before sending them to it, but control the overall level of your drum mix using the bus fader.