You really don't need to register as a business until you get in bed with labels, A&Rs, and musician's unions...You know, the people in suits.
Sales tax still does not apply to online sales, I think, so you are not obligated to pay any taxes as a "business", which would really be the only thing you would have to look out for.
You can go under the table for a long while, but if you want to secure some sort of name, I would look into filing a DBA ("doing business as"). This is the cheapest way to operate under a legally-secured name, and I don't think it requires any sort of monthly/yearly payment.
Otherwise, just network with others, and always look for ways to step up your hustle.