sure, just run a cable from the audio output sockets on your direcTV box, to the audio input socket on your mac.
then you'll need some sort of recording software. you might have "sound studio" supplied with your mac - i had it pre-installed on my machine. if not there are tons of programs on versiontracker.com
all you need to do then is connect the cables up, open the recording software, and hit the "record" button in the software. it'll be recording in CD standard sound, which will take up approx. 10 MB per minute.
once you've recorded your audio stream, you can make any necessary edits, fade-ins, etc. then import the resulting AIFF files to iTunes and convert them to MP3 or AAC.