pro studios will always listen to their mixes in mono to check the levels of each individual channel in the mix, this is because it really gives you a good idea of balance of the frequency spectrum, it also gives you a good idea of what you will hear on a crap mono radio. i recommend this to anyone struggling with their mix downs in their home studios because it is a great way to learn how to balance you instruments with in a mix, particularly drums. there is the struggle of choice in the pro production industry whether you go for the highest quality broadcast systems available or simplify to reach a greater audience. however this is not such a huge issue as it was ten years ago with the price of computer surround speakers being available for the price of two cds.
stereo, 5.1 and up allows you to physically move the signal from the centre (where your mono all sums to) to anywhere you want in the physical space of your speaker setup. this also allows you to spread the same signal over multiple output channels such as left centre right sub rear left and rear right.
in the live pro audio world, what is happening is numerous mono signals are input into the console and the engineer then makes the decision where these signals should be sent in either the stereo field and beyond. i have worked on professional theatrical productions where i have had up to 24 different places to be able to output signal to. this is quite complex but can be exceedingly satisfying to hear what can be achieved in a live situation.
i hope this gives a bit more insight into the world of audio and answers your question at the same time.
TP
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