All the relevant code currently (FreeBSD 4.4) lives in /usr/src/sys/dev/sound/, except for the public ioctl interface definitions, found in /usr/src/sys/sys/soundcard.h
Under /usr/src/sys/dev/sound/, the pcm/ directory holds the central code, while the isa/ and pci/ directories have the drivers for ISA and PCI boards.
This, and other documents, can be downloaded from ftp://ftp.FreeBSD.org/pub/FreeBSD/doc/.
For questions about FreeBSD, read the documentation before contacting <questions@FreeBSD.org>.
For questions about this documentation, e-mail <doc@FreeBSD.org>.