Does My Husband Need to Cancel His Media (M) Visa to Apply for a Dependent Visa in Thailand?

Okay, you say he is on a year extension from a Type M Non-Immigrant visa.

I do not know whether he can cancel the current extension of stay based on employment for a media company AND then immediately apply for a new yearly extension that matches yours because he's the spouse of a foreigner who legally works here. :/

In theory he should be able to. He'd need documentation from who ever gave him the paperwork to get a year extension stating he is no longer 'employed' by them as of xxx date. he'd need documentation from your company stating that it's okay he goes on a trailing spouse extension, and he'd need the marriage certificates, your contract, passport work permit etc.

I'd say BOTH of you should go out to the immigration office well before his current extension expires and see if it can be done AND if it can, get the hand out that lists the requirements.