The best thing to do IMHO, is prepare for the shoot beforehand. I know a guy that used a laser pointer to get the video synced. Just point it to where all of the cameras can see and a quick click and note when you did it. Then in editing just line up the flash and your good. I've also heard of come kind of clicker (makes a sharp audible click) shows up on the waveform bar really well. In both instances you want to make these marks in places that you can cut out of the final video.
At least that is what I think.
Now if you've already shot the video. It will be a lot harder. maybe pick places like the beginning and/or end of songs.
I understand that there is a multicam plugin somewhere around that might make editing easier. I don't really use speed edit so I don't know much about it.