Good musicians play in time and play in tune. In order to do that, they need to listen to and hear themselves and all the other instruments, including the vocals.

Each instrument must be played at a suitable level and occupy a separate frequency. If the drums are too loud play them quieter. Don't turn up the guitar so you can't hear the vocals.

Use a metronome to practice with, in order to master the art of keeping in time and a tuner (all use the same tuner) to make sure everyone is playing in tune (and in tune with each other).