TLDR: I could not get my CT4780 to let me enable Line-In and CD Audio at the same time in the Windows audio mixer. I discovered changing the value of "PlaySources" buried in HKLM/Software/Creative Labs/Drivers/DefaultMixer from 0x06c8dc to 0x04c8dc allowed me to. My next task is to get the PC Speaker mixing to work.

Long Version: The CT4780 is a pain in the ass model of the SBLive. I never exactly know how I get it working from install to install. The most common failure mode is you go to install your recently downloaded SBLive drivers, they look like they install, your mixer shows audio is playing, and yet you hear nothing. Some combination of installing the Liveware 3.0 drivers and a random hacked together driver set I found on MSFM eventually get it mostly working. However, passing through assorted driver versions that worked to varying degrees on this rather frustrating OEM card, sometimes the PC Speaker works in the mixer, sometimes it doesn't. Sometimes I can enable every single input mixer, sometimes I'm restricted.

Anyways, after some trial and error, and comparing registry settings in INF files, I eventually figured out the setting above for "PlaySources" that enables you to turn on all your input sources at once, to no apparent ill effect. For me this matters because I run my system with an SBLive and an AWE64 bridged to the line in of the SBLive. So it's nice to have CD Audio and Line-In turned on at once for those DOS games with SB16 sfx and CD Audio. It's also nice not to constantly have to fiddle with the volume mixer, deciding on a game by game basis whether I need my AWE64 unmuted or CD Audio.

My next goal is to get the PC Speaker turned back on. This is less necessary because it works fine in DOS. But, because I'm stubborn and refuse to give up, I want it to work in windows as well. My problem is the mute and vol controls are just greyed out disabled. Once again, through assorted driver versions, the enabling of these has come and gone. I've tried setting the PSpkMt value in the DefaultMixer registry settings, values of 0x000000 and 0x010001 being common, but neither changes anything. I've also noticed some variance in the settings for PSpkVol from INF to INF, or even with INF's depending on specific PCI device, but the value in my current registry bares no resemblance to the INF setting. So I have to assume that value is the literal vol level currently, and not some sort of enabling/disabling series of bit flags.

I never saw this documented anywhere, so I figured I'd throw it out there, for those few other weirdos struggling with this very specific problem to eventually find it.