The California Courts official website has a self-help guide to traffic cases, where you can download instructions and the necessary forms to appeal your conviction.
I'd also suggest you invest in a copy of "Fight Your Ticket and Win," by Nolo Press. It's available for about $25 at most bookstores and online retailers.
Before you spent a lot of time and effort, you need to start by understanding what an appeal actually does. Appeals are based on legal errors, which tend to be fairly technical.
On appeal, the reviewing court does not reconsider the facts of the case. In your situation, you can't successfully appeal the case by claiming the cop was lying. The judge who heard the case heard from you and the police officer, then decided he believed the cop, so you're stuck with that decision.
For instance, if the officer said he was 30-40 feet away, and your stereo was blasting so loud he could have heard it much farther away, that would be enough to convict you if the judge believed it.
I also don't understand your statement about having the top down. If your windows and the convertible top were down, the sound would travel much farther.
Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like this, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.