I can't address the low-volume issue but I've got a few CT4600-Series SB LIVE! cards that work on Win7-64.
However, I didn't do Add-In's later - I had those cards installed from the beginning. SB LIVE's driver offering might be scattered all over the web - you might Google "SB CT4670" and see if you find some sites. Here's an old one, offering Soundblaster's own drivers - but from 2005 and only for XP-64 as a Beta.
To disable onboard sound, first I'd go into Windows Control Panel and see if you can Uninstall all Realtek Audio drivers. Then, without rebooting, see if you can DELETE REALTEK SOUND CARD. At this point, you're probably forced to do a Restart.
Do a Shutdown instead. (You don't want Windows restarting and 'see' the audio chipset and reinstalling the drivers-that-don't work. Not yet, at least!)
Then, upon reboot, enter the BIOS SETUP (DEL or F2) and find the options to "onboard audio = DISABLE". The option may be called "onboard audio" or "sound chipset" or ?? And it may be tucked away into some subsets of the TForce BIOS screens. Sometimes I take a note or two to help me remember what I've done. But there will be an Onboard Audio option that can be disabled.
SAVE this BIOS setup and, when the computer restarts, do a Shutdown fairly quickly - long before Windows starts up.
Now, install the old sound card.
(There is a chance that inserting the soundcard will auto-disable onboard sound, by the way. But since it's an older chipset, I'm not sure if the TFORCE motherboard will happily disable a new Realtek for an old CT board.)
If Windows can't locate drivers itself, ugh. You might need to Win7-32 Install disk. If it still can't find drivers, and your earlier search for them was fruitless, then I'd consider "Do I want a full backup of my files and a clean re-install of Win7-32? Or try to find some solution to the Low Realtek Volume."