If you haven't already I suggest looking at Neuronix's stickied guide about applying MD/PhD at the top of the board.
In general the main part of your application (provided your GPA and MCAT are Ok which it seems like they are) is research. The impression I have garnered from this forum and from events at my University is that research will be far more important than anything else on your application. Volunteering and clinical experience should still be present and you should have a compelling reason for wanting the MD and not just the PHD but those factors will not contribute to your application in the same way that a thorough knowledge of your research field and a demonstrated commitment will.