Yes this does seem to be an issue. I believe you can move your start point, just not forward. I have started later in picture, but that was many years and Sonar versions ago, so I'm not so sure.
I always prepare my video in a video editing app (a Sony Vegas lite) as
dlion16 above recommends. I always have 10 seconds prior to where music starts just for safety sake. I have been doing that for years and I have no complaints about how anything works as far as Sonar is concerned. I just load and go compose.