Do I have to put up with my coworker's horrible voice? I'm going to tell them to stop singing at work
Do you really want him to stop singing?
Yes, I hate him singing at work, he sings off-tone
You shouldn't try to ruin someone's good time. They may get upset if you try and tell him what to do.
Work is stressful. I just want to relax and listen to music. I would rather ask him to stop singing.
They're your coworker so you should be considerate enough to  understand their feelings. If they're in a good and singing a song it's alright. It's important to get along with coworkers to maintain a healthy work environment.