It's highly unlikely that a connection from your scanner's audio out (either speaker or line out) would do any significant damage; there isn't any voltage or current (to speak of) on either.
Cheap audio cable - such as the stuff you see at RS - will work, but if it's of any real length, might act to pick up noise and other junk and route it into the soundcard. Check out something say from Best Buy, or get some decent audio cable from a stereo store and build your own. It's quite easy and should pose no read problems to make. Alternatively - and I know a couple that have done this - I understand RS still sells a kit with RF chokes to try to cut the noise. If you find you are hearing junk from your PC in your connection, this is an easy solution. 73s Mike