During this phase, the musicians try out their instruments, and while this happens the two sound engineers work on the gain-levels and the tones of the sounds coming from the stage. In smaller venues this operation can be done "invisibly" by the sound engineers: the musicians are be left free to practice their pieces without interruptions or conditionings. But usually every sound coming from the stage has to be manipulated individually: each musician is asked to produce single sounds from his/her instrument. For example the drummer will repeatedly beat every component of his drumkit until both the engineers are satisfied of the manipulations they applied to each single sound and move on to the next component. When bands have a big management behind them, this operation, which often takes a lot of time since has to be done on every single signal of the stage, is carried out by technicians and not by the actual members of the band which pop up when all signals are ready and is time to step into the next stage.
It's time to fully play some of songs to setup the FOH and the stage mixes. The band starts playing and the stage engineer, having the chance to listen to the sounds he is sending through his headphones or through a dedicated monitor next to him (the stage mixers have a SOLO system [SOLO system control ] which allows the single aux send master to be listened to), creates some initial mixes based upon general criteria. For example, in the drummer's mix he will prominently send the bass signal and perhaps the singer's voice, whereas to the singer's mix he might send mainly the singer's own signal, seeing that he or she needs, above all, to be able to hear him/herself, and with the addition perhaps of a bit of reverb.
Starting from these basic mixes, each musician asks the stage sound engineer to make any desired corrections and changes to the sound, and the latter (usually :-P) satisfies these requests. During all these phases, the front-of-house engineer takes care of the sound that comes out of the P.A and constantly tries to improve it. By the end of the soundcheck all the levels will have been calibrated, so all is interrupted leaving everything as it is (the equipment is all left turned on and all the channels are muted) awaiting for the concert to begin.