I think what you're suggesting is very interesting, but essentially useless. Creating music every month for new themes would be exhausting, tedious, and overall, not worth the effort. To create a staff group for that purpose would be a waste of staffs time and effort.
However, a group could be created outside of staff that could make music and sound design for the game whenever they feel like it. As such, this group and staff could work together to implement new sounds and music for any updates to the game. This way, there's no deadline, staff isn't directly involved which means anyone could create music that could be potentially used in new updates and any faults this new group makes don't fall on staff shoulders. I would make music and hit sounds if this idea went through, I don't have any problem with doing that for the community.
Tldr: staff group bad idea, could be cool for organization though.