The speakers do have volume control. As far as I can tell their cables have 1/8" jacks which plug into the sound card. I don't know whether they will also work with a line out. The TV has a red and a white out (and a yellow, which I understand to be for video).
I bought an RCA-to-1/8" Y cable (from RadioShack) and it worked. The Y cable has two RCA male jacks (1 red & 1 white) at one end, and a 1/8" male jack at the other. (I also bought a 1/8" to 1/8" F/F connector because I couldn't remember whether the speakers were male or female. As it turned out the speakers have a female 1/8" jack, so I didn't need the F/F connector.)