So you are using Mandrake 8.1 again now, and this time the card isn't working? Correct?
modprobe is used from a shell as root, it should be located in root's PATH so you don't have to worry about which directory it's in. However, if you are curious you can always locate binaries with:
And it will locate the file for you.
In your Mandrake Control Center, you have a hardware section. Go there, is your SoundCard listed? If so, then you might just need to unmute the volume. If it is, but it's listed as not working, you may just need to configure it. Click "configure" in the bottom portion of the screen once you highlight the device.