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In general, without auxiliary specifications, you can use AC3 in VBR mode too (I believe Aften supported that). But there are consumer media specifications (e.g. "DVD Video") which require CBR audio streams. AC3 can, but is not allowed to under certain circumstances.
For audio formats with a good "channel coupling" (known as "Mid/Side" encoding for MP3 which supports stereo at most, but it can be handled in a similar way for more channels too), there is a theorem that the bitrate shall be in relation to the square root of the number of full frequency channels to achieve similar quality. It usually works quite well for AC3, comparing a 2.0 bitrate as √2 times a theoretical base mono bitrate with a 5.1 bitrate as √5 times the same theoretical base mono bitrate. Depending on the content.
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