"DougSlug" wrote in message
...
How can these SCA transmissions be decoded? Are there any sound
card-based software techniques, or does it always require hardware?

Cannot be done with sound card or software. These are subcarrier s in the
FM signal. One has to tap the receiver after the demodulator and send
through a separate LF FM receiver ("adapters"_. Note that there are
legalities to consider in the US.

The site mentioned
http://members.aol.com/fmatlas/home.html
sells modifired FM radios and adapters.