The two biggest issues that come up when clients send vocal files they recorded at home are distortion and room noise. Hard surfaces create reverb reflections that bounce all over the room and back into the mic. This makes a real unpleasant “reverby room” sound that you can’t take out after it’s been recorded. The best way to get rid of that is to add soft surfaces. That can be as simple as adding some carpets and couches, or you can get a bit more advanced and put up acoustic panels.
The other problem is distortion. I think the main reason this happens is artists can’t hear themselves and instead of turning the music down, they turn the gain of the mic pre up thus creating distortion. Check out the step by step video on how to avoid this, and please let me know if you have any questions.