Michigan's Department of Licensing and Regulatory Affairs (LARA) has posted a document titled 2015 Michigan Residential Code Errors and Conflicts ,which includes its resolution of a conflict regarding the use of building framing cavities for plenums.  In summary, the resolution states:

"The Michigan Residential Code (MRC) Sections N1103.2.3 and M1601.1.1 conflict regarding the use of building framing cavities for plenums.  The conflict is resolved in favor of M1601.1.1 as MRC Section M1601 is the definitive section on duct construction.  This decision is based on MRC Section R102.1 (Where there is a conflict between a general requirement and a specific requirement, the specific requirement shall be applicable.) and that Section N1103.2.3 has not been shown to meet the definition of cost effective."