Don't wanna sound biased, but there are a lot of ways of dancing live without showing vocals mistakes besides total lip-synch... And if even so any performace was bad, a live DVD can be totally edited and the voice will sound perfect^^
And ayu used all the stuff you can use to make your voice sounding good live... lip-synch, back-track, louder back singers, loud back singers + back track, vocal replacement on dvds, vocal editing on dvds, louder instrumentals...
And there are softwares that fix some singing mistakes totally live... In the end, the only way to see how well someone sings is by listening he/she live, in person, acapella and without microphone
