It is very rare to be able to take pictures during sound checks before some big concert. Usually musicians will not like anyone on stage with a camera and flash jumping around and taking pictures. I've done it before and conditions for doing it are not ideal, as the light is not turned on, there are lots of people all over the place preparing things for the concert, there's always a lack of time and everyone wants to finish it as fast as possible. Good guys of Lačni Franz, a legendary Slovenian band from Maribor allowed me to join them on stage before their appearance in Cvetličarna where they played with Mi2. I think the whole sound check with most of them on stage lasted about 2 -3 songs , so overall, the same timing as normal concert photo shoot.