The rules around volunteer work under federal and Texas law are not very complicated but commonly misunderstood. Under the Fair Labor Standards Act you cannot volunteer for a for-profit employer performing work for the employer. (Aside from a few professions, such as lawyers and doctors, and exempt employees performing exempt work.) No matter how badly you might want to do the work as a “volunteer” you cannot be a volunteer. The employer must pay you minimum wage and overtime pay (if you work the requisite hours). This is true whether it is your regular employer or some other business.
Non-profit organizations and government agencies can accept volunteer work so long as the work is not unpaid work that the employee/volunteer normally does for pay for the same organization. If you work for a non-profit, you cannot work for pay 40 hours and then volunteer 8 hours.
However, these basic rules work within the context of a couple minor rules. First, volunteer work must be truly volunteered and not coerced. For example, if you work at a company and they sponsor volunteer work on the weekends, they do not have to pay you for doing volunteer work, even though it creates a good image for the company. The company can encourage volunteering; but if volunteering becomes a requirement to keep your job it is no longer volunteer time and your employer must pay you for that time.
Second, some work performed for the employer’s benefit isn’t employment that requires compensation. If you do something of your own free will that provides a minimal benefit to the alleged employer and it is not part of conditions of existing employment with the entity it likely is neither volunteer work, as defined by law, nor is it employment. For example, if you love a restaurant and spend your free time telling people how great the restaurant is, that is an incidental benefit to the restaurant’s business that you are doing of your own free will. You likely have not earned wages nor are you the restaurant’s volunteer.
Volunteer vs. unpaid interns
For-profit employers can get themselves in trouble when they accept “volunteers” or “unpaid interns” looking to break into a particular industry by letting them do the grunt work without compensation. It is employment and subject to the Fair Labor Standards Act. The same happens when employees grab an opportunity for experience because they “volunteered” to work without pay. It is not volunteer work under the law. It is against the law for the employer to not pay the employee.