and I can enable the optical spdif output with
# sysctl dev.pcm.1.spdif_enabled=1
I always use this to put music on my ac3 receiver and it has never disappointed me in any way. The quality is very good, what isn't supprising, because the audio stream is 'only' put through the optical wire.
Furthermore I got a recommendation that the Audiotrack cards have the same ability, but I can't verify that. The new hda driver, theoretical, provide a new device in purpose to convert the signal to put it out on spdif. I'll try that in future. I have no ideas about your cards. I never try them.
If you want spdif output on FreeBSD, I suggest you buy a Terratec for arround 20 Eur (~25-30 USD I guess).
I have a Zoltrix Nightingale Pro 6 also using the CMI8738/PCI-chip. With the "builtin" sound (described by s-tlk above) I didn't get SPDIF output to work, but with OSS from ports it works perfectly.
One thing I especially like is the automatic change of output signal frequency between 44.1 and 48kHz depending on the source. Previously (in linux) i always had to change that manually (or suffer quality degrade).