Depends on exactly how loud it is. There are noise ordinances everywhere, so you can get charged for violating them if your sound system is loud enough. And a lot of jurisdictions also have specific regulations concerning music coming from cars (though they are typically formulated to be consistent with the local noise ordinances). The standard rule for those jurisdictions that have specific rules about car stereos is that the music should not be loud enough to be heard distinctly (e..g., someone could make out the lyrics) from 25 (sometimes 50) feet away.
When the law is enforced, the penalty is typically a ticket with a graduated fine (e.g., $80 for the first offense, $125 for the second). In jurisdictions where the law is more lenient (such as where they use the 50 foot rule) the starting fine is usually higher (so $125 for the first offense and $250 for the second is likely). Since you have to be more of a nuisance to get fined in the first place in those jurisdictions, the penalty starts out more severe. But like all local laws, it can vary from place to place—so you might want to look into it if you are worried about being reported (or wanting to report someone).