hi im using fl studio 10 (extended memory) with my built in sound card , the problem is when i listen to the songs i rendered i notice that some instruments cant be heard because they're too low even after i work on the mixer withe every channel to hear everything clearly i must say that i dont put anything on the master channel and i have to put the slider all the way up in some instruments because i cant hear them in the mix that makes them sometimes hit the red zone in the meter ... i dont know

Could be an issue with your soundcard, or you forgot about your speaker volumes? It can happen. If you can hear it all fine in FL, it should sound normal outside of FL.