Is the worker sinful if he works for a voice services company that sometimes provides its services to wedding halls that are not compliant (with musical instruments and so on), knowing that his work falls under the inclusion of the ISO 9001 certification?

If your work does not directly involve assisting in sin, there is no harm in you working for your company, even if the company provides services to forbidden entities, unless your work is related to that. However, those in charge of the company should be advised to avoid serving forbidden activities, such as wedding halls where mixing, dancing, and musical instruments are prevalent, due to the Almighty’s saying: "And do not cooperate in sin and aggression" (Al-Ma'idah: 2).