The following would be your card, indeed no driver currently claims it, yet.
Code:
none0@pci0:0:31:5: class=0x040100 card=0x434d1849 chip=0x24c58086 rev=0x02 hdr=0x00
vendor = 'Intel Corporation'
device = 'Realtek AC97 Audio (82801DBM SoundMAXController (ICH4-M B0 step))'
class = multimedia
subclass = audio
This
handbook page indicates that
kldload snd_driver will attempt to load several common drivers, did you try that?
A full list of sound card drivers can be found using apropos or man -k on a FreeBSD system, but
here is an online list.
My bet is
snd_ich(4) or less likely
snd_hda(4).
Hope that helps, but, it seems Beastie got here first.