I'm assuming that by "cd/mp3 system" you mean audio review products, if so, here are the ones I know of:
- Gold Standard Audio for step 1: I haven't tried it. From their website I found out that the CDs cover all of the subjects for Step 1 in Q & A format.
- Goljan audio: floating around online, lot's of people use it and most people think it's great.
- USMLE Step 1 Recall: Buzzwords for the Boards (the CD not the book): I have this and I think it's great for listening to after you've finished each subject. It has about an hour worth of question and answer review for each subject. I recommend this.