@monkeyonthetable that sounds like someone who understands the transmission risk! That could be considered safe.
@Rollawayyourstone you're right to be worried about singing - massively increases quantities of airborne particles which hang around rather than fall to the ground after 1.5m so I wouldn't be keen on this unless it was outside.
General note about air con - the biggest risk is when airborne particles build up in an area. A minor dose of airborne is unlikely to be enough to give you COVID but a good lungful of a concentrated amount will. Airborne particles are generated through breathing through the mouth, singing and talking loudly. So you want to avoid anything which creates a lot.
Then you need to try and get rid of them / disperse them as quickly as possible.
Hence why outdoors is so low risk - the particles are in such a low concentration so quickly it doesn't matter.
But an airless, unventilated, low ceiling / small room space for a few hours will just build up and up and up. It won't matter if you are 6m away. You could still be breathing in a heavy load.
Air con - provided it is a fresh air system, isolated that brings in outside air through one room and back out again is A Good Thing as it helps to lower concentration of particles.
Air con that recycled air or uses the same air to go through many rooms is the danger zone as it will just spread particles without removing any.