What are the best ways to look for live concerts (mostly classical, but I'm open to local genres of music as well) while we (husband and I and daughters ages 17-10) are visiting? We'll be near Frankfurt (in Mainz) Dec. 10-17 and Munich Dec. 17-21.
For classical music the local and regional visitor and event websites are in most cases the best source.
For Frankfurt: https://www.visitfrankfurt.travel/en/experience/calendar-of-events
Event provider Eventim is also a good source. Example for a classic event in Mainz:
https://www.eventim.de/en/event/der-nussknacker-grand-classic-ballet-die-traditionelle-wintertournee-rheingoldhalle-mainz-20262762/?affiliate=TUG
I guess it is not your preferred music style but Frankfurt has traditionally a strong techno (club) and German rap scene. Moses Pelham is somehow a local hero and is producing a wider range of music nowadays.
Frankfurt has traditionally a strong techno (club)
Sven Väth and The Omen club were very influential globally back in my day.
I often look up churches where I am going to be and see what concerts they have listed on their website. This gets you smaller events that are sometimes free. Like this in Munich:
https://www.muenchner-dommusik.de/kalender/kalender-dommusik/?tx_cal_controller%5Bgetdate%5D=20251126&cHash=f8aed1613633c494f55f4ae2a7400f05
There’s a short free organ concert on Dec. 15 & 19. I found a couple of interesting concerts on the 21st but looks like you are leaving on that date.
I just did a 10 minute search, first on Google maps (zoomed in on Munich) for cathedrals, then their websites, then Music or Concerts.
Or I searched opera house & came up with The Night Before Christmas at the Opera House in Munich on Dec. 19. There aren’t many tickets left, but they are only €10 (might have some line of sight issues, but for €10/e might be worth it just to be in the opera house).
https://www.staatsoper.de/en/productions/die-nacht-vor-weihnachten
All of the downtown churches in Frankfurt have concerts almost every night during Advent. Organ, choir, bells, medieval instruments, brass, and so on. The Kaiserdom, Alte Nikolai, Liebfrauen, and 3 König. Mainz may be similar.
Here's the list for St Stephan in Mainz, which you should visit if only to see the Chagall windows.