The only rules that matter are the ones at the convention at which you plan to enter. Read them, they should be on the con's website. And if not, email the con staff/cosplay staff/cosplay director, who makes the rules.
Rules differ wildly from con to con. Most cons where I spend my time? Don't allow you to actually sing or deliver any dialogue at all from the stage. It's too complicated and expensive to have microphones on stage and a sound crew who can properly and intelligently manage said microphones, so MOST cons require you to pre-record everything and the only thing you can do on stage is lip sync. But, the only way to know for sure if the con you're planning to perform at also has this rule or if they do things differently, is to read THAT con's rules page. And if they don't answer your questions, email the director.
Founder and Vice-President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com