Is it feasible to decode an 15 kHz RGB (SCART) signal using a sound card?
Let's have a look at the VGA (RGB) timing. A good intro is here: http://www.xess.com/blog/vga-the-rest-of-the-story/
So, a 'line' takes 25.17us. Assuming, say, a 800x600 picture, that's 25.17/800 us per pixel. Assuming an ideal Shannon limit, you'd need to sample at at least half that time to recover the signal. So you sampling frequency needs to be at least 1/(12.6e-6/800) Hz, which is 63MHz.
From Wikipedia:

For most common VGA mode 640x480 &quot;60 Hz&quot; non-interlaced the horizontal timings are:

Pixel clock frequency 25.175 MHz
Horizontal frequency 31.4686 kHz


So, not going to work I'm afraid.