Let's look at the difference between reasons and excuses here, when you should mention something and when you should keep it to yourself.
- FOR REHEARSAL: -DO tell the creative team if you're sick before rehearsals start, otherwise they'll wonder if they did a mistake by casting you in the first place. A lot of singers don't tell because they're afraid the creative team will think that they're not reliable, but it's an important piece of information for the creative team to know why you're not giving your whole. DO tell if it gets worse in the middle of rehearsal, and if you're getting a fever, because you might be becoming contagious and you need to go home immediately and take care of it. Everyone will appreciate you for it. DO NOT tell if you're fatigued from being sick if it makes no changes to your sound or to what you're physically able to do.
- FOR A LESSON: -DO tell if it impacts your sound. DO NOT tell it you think that's a reason for not coming in prepared. There's a lot of mental practicing and table work that can be done even when you're sick.
- FOR AN AUDITION: -DO NOT tell BEFORE you sing, because if you happen to sound great, people will think that you were just trying to cover for a potentially bad audition, and they might think that you are insecure and that you may be a pain in the butt to work with. DO tell right AFTER singing if you really didn't sound normal, and apologize by telling what it is you have. Don't say: "Sorry, I'm kind of sick," but "I hope I get another chance to sing for you again because I have a terrible cold which is impacting my sound negatively." If you didn't say it at the audition, DO NOT email the casting director about it, you missed your chance.
YOU DIDN'T HAVE TIME TO PRACTICE
- FOR REHEARSAL: Unless something major happened such as you had to take your child to the doctor, or your car broke down, or something tangible like this, no excuse will ever be good enough. Because most likely the reason you didn't practice was not because of a lack of time but because of a lack of organization. You could have practiced at some point but you thought you could practice later, and then something came up, and here you are, not prepared. Practice when you can.
- FOR A LESSON: There's no really good reason for not being ready for a lesson because you have a whole week to get ready for it. It's even worse if you try to fool your teacher saying that you practiced when you didn't, so the best is to tell right when you walk into the lesson that you're not ready and that you're sorry (and you better be sorry because you're wasting your teacher's time and you're throwing your tuition money away).
- FOR AN AUDITION: No excuse or reason will ever be good enough for you to go to an audition not prepared. If it's a call back and you do have a good reason for not being prepared, then call ahead and cancel your appointment. If you have a relationship with the creative team, you may email beforehand and explain what your reason is for not coming to the call back, and that you don't want to waste their time by showing up unprepared. They might give you an appointment for a later date, but otherwise, know that you did the right thing and that they will respect you for that.