Maybe the judge will have a good day and allow that volunteer work. But, I am not thinking that one is going to fly.
Usually, when an agency agrees to participate, they complete paperwork verifying your hours, etc. They may even agree to send in the reports. You probably have none of that where you are volunteering at present.
Again, you seem to have decided to take it upon yourself to determine the terms of your sentence with respect to the community service portion. You have ASSUMED that what you were doing would be OK. You know what they say when you ASSUME something. You make an a** out of you and me.
In your future dealings with the court, I recommend that you be much more proactive.
Firstly, if you still want to complete your community service at the agency for which you currently volunteer, verify that both this agency and the court will accept this agency before making any further commitments.
Secondly, as Ford stated, court orders are not "suggestions".
Finally, if it so happens that you actually enjoy the community service work at the assigned agency, consider it a bonus. But, don't go with the idea that you will or that you can change your mind and go off and work somewhere else without any notice to anyone. The system is almost certainly somewhat rigid and bureaucratic. If you don't dot your "i's" and cross your "t's", count on problems.I would recommend that you get a copy of any required paperwork that the agency may send about your service. Paperwork gets lost all the time.
Since there is a possibility that the judge could throw you in jail, you probably can ask for the assistance of the public defender at any forthcoming hearing. If you do get additional time to complete the community service, you need to understand that the judge is calling the shots. The burden is on you to toe the line as ordered and not somehow believe that an exception exists because you don't like the assigned community service work, etc. That ain't the way this works.