Part of the reason for hissing is how the volume sliders are laid out in Windows mixer and in SETUPSA utilities. The 0db points of the input lines are at the very first notch in Windows mixer and not at the top. Moving sliders to top will cause up to +12db boost which will also boost any noise. Worst offender is the Synth/MIDI input which is very noisy due to OPL3 DAC inside the chip not being anything great.
Master, Wave and Software WT should be set to max, Synth, Line, Mono in, CD and Mic should be set to first notch and all unused inputs muted. This will provide best SNR.
In pure DOS you'll want to use my SETYMF utility instead of SETUPSA as it provides much more control and is more user friendly, you can find it here : http://www.tmeeco.eu/TKAYBSC

Other reason is that you most probably have sound output going through the speaker amp, which will be very noisy too. There's two jumpers in top corner which will select if the sound is direct (Line out) or through the amplifier (speaker out).

For those who are interested, I was able to figure out what the two 4-pin connectors at the top of the card are (LGRG and LGGR). These are the CD audio in. It looks there were two standards.
L = Left
G = Ground
R = Right

The 4-pin CD audio cable (MPC2 -- Multimedia Personal Computer 2) that is connected between my DVD-ROM and Sound Blaster Live! 5.1 is wired for LGGR. (The white wire is Left, black is Ground, and red is Right).

Many sound cards have two connectors because one is for the CD/DVD drive and the other for routing the modem audio in/out via the sound card.

So, I'm able to to verify that the sound is being processed by the Yamaha in both DOS and Windows 98, I don't know what kind of magic that took place, but I do get sound out of the 3.5mm jack.

I am also able to connect the CD Audio out of the DVD ROM into the 4-pin LGGR connector.

And, yes, it looks like the 26-pin connector is for the Wavetable Daughterboard.

More Questions
In Windows 98, I'm able to process the sound on the Yamaha and send it out via the line-out into the line-in on the Sound Blaster.

In DOS, I'm either only getting sound out of the Yamaha or both. But, not able to get sound out of the Yamaha and pass through the Sound Blaster.

The Yamaha drivers were installed in Windows 98 and the card works fine when I "Restart in DOS".
The Sound Blaster drivers needs to be initialized in DOS in order for it to work. I'm using the drivers posted on Phil's Computer Lab.

How do I replicate my Windows 98 setup -- allowing the sound to be processed via the Yahama and passed through the Sound Blaster?

Why Am I Doing This?
Ultimately, I want to have my Sound Blaster Live! 5.1 handle sound in Windows and my Yahama handle sound in DOS. But, I only want one speaker connection. If I can pass through to the Sound Blaster in DOS, I won't have to switch inputs on my external speaker.

w0lf42 wrote:

How do I replicate my Windows 98 setup -- allowing the sound to be processed via the Yahama and passed through the Sound Blaster?

You need to use Sound Blaster Live's DOS mixer utility and mute all channels for that card except Line In

Thank you for the advice. I found this Multichannel DOS Mixer which did the trick.

w0lf42, if you need only one input you may use Creative's DOS driver and mixer. It enables CD IN. But you'll must load this TSR with EMM386 always. My mixer can work with Creative's TSR driver or without it, with EMM386 or without it, can enable/disable ANY input and route it to ANY output (dig/analog), even (for example) so:
# LINE IN2 ---> 45% VOLUME FRONT OUT, 23% VOLUME REAR OUT
# CD SPDIF ---> 100% VOLUME REAR OUT, 10% VOLUME FRONT OUT
# WAVE ---> 80% VOLUME FRONT OUT, 0% VOLUME (MUTE) REAR OUT
- and all this simultaneously, any front-rear fade panorama is possible separately for different channels. Note that AC97 input channels are routing only together because they are ONE input digital channel for EMU10k (see schemes Multichannel DOS mixer for DOS sound builds v0.49 for Live, v1.01 for Audigy).

Is there any way to have both the ac97 line and ac97 cd on? I'd really like to have two analog inputs.

As far as I can tell, I can turn on line:
ek1m -as line
or turn on cd:
ek1m -as cd
But not both.

Thanks

I did some more reading the documentation and trying options. I'm sure I'll refine this, but this works.

1ek1m -b
2ek1m -as mix
3ek1m -ai aux 100
4ek1m -ai line 100
5ek1m -ai cd 100
6ek1m -io ac97 100 100 100 100 100 100

Thank you again for this software!