I'm attempting to move the signal from my Yamaha MU80 into a different input

Fair enough

According to the 40-pin header on the sound card, pin #27 is a MIDI Input.

That input expects digital MIDI data (from the MIDI out of a MIDI device), not analog audio .

According to the 40-pin header on the sound card, pin #27 is a MIDI Input. So, I (1) made a MIDI to 3.5 mm headphone cable, (2) connected that to a female 3.5 mm headphone jack, and (3) connected the pins on the headphone jack to the correct pins on the sound card header.

If you are trying to route analog audio from the MU80's headphone jack to pin #27 on the sound card header, that won't work as that pin expects digital MIDI data .

If you want to get analog audio from the MU80 into the sound card, you need to use an analog input like the analog CD IN or the AUX in on your sound card .

Sorry, I see how that could seem like I'm doing analog out. The cable is a 5-Pin MIDI cable to male 3.5 TRS. It's not converted to analog -- I used the newly released TSR MIDI Cable Standard (http://cdm.link/2018/08/midi-minijack-trs/). So, it's still MIDI, just a different connector.

Sorry, I see how that could seem like I'm doing analog out. The cable is a 5-Pin MIDI cable to male 3.5 TRS. It's not converted to analog -- I used the newly released TSR MIDI Cable Standard

OK, so if I understand correctly, you are trying to send midi DATA from the MU80 back into the PC through one of it's MIDI inputs ?
Is the end purpose to record the MIDI data back into a MIDI file ?

If that's the case, a joystick-to-MIDI adapter would be enough, as that has MIDI In. However, I read analog audio all the time, and 3.5 mm headphone jack, so...

Anyway, the wavetable connector is TTL, whereas MIDI uses current loop. You can't just make a direct connection between then.

Question
Cab I use the 4-pin CD/DVD audio-in (LGGR) as an analog input from another source?

Background
Okay. So, I'm still looking for solutions. I've learned a lot (and I'm still learning). I'm assuming that the Yamaha MU80 outputs in either MIDI or analog. Since MIDI is only an instruction set, I want the Yamaha MU80 to process it and output it via analog. I only have one analog input on the Sound Blaster Live! 5.1 and that is currently being used by my Yamaha YMF719 sound card. The Sound Blaster has a 40-pin header (I assumed to be connected to the 5.25 bay drive). I looked at the pinout (see below), and can't locate an analog in.

Things I've Tried
I've tried running the processed sound from the Yamaha MU80 through the Yamaha YMF719, but the sound volume is lower and, I assume, lower quality.

I've tried using a 3.5 mm y-splitter cable -- combining the sound from the Yamaha YM719 and the Yamaha MU80, but that just mutes the sound from the Yamaha MU80.

Sound Blaster Live! 5.1 Pinout
01 VCC +5V Power Supply
02VCC +5V Power Supply
03 GND Ground
04 AC97CLK 24.5 MHz Clock Output
05 GND Ground
06 GP_SPDIFIN#2 SPDIF Input Signal
07 GND Ground
08 GND Ground
09 SPDIFO#3 SPDIF Output Signal
10 GPO1 General Purpose Output
11 GPO2 General Purpose Output
12 GND Ground
13 GPO0 General Purpose Output
14 GND Ground
15 GP_SPDIFIN#1 SPDIF Input Signal
16 GND Ground
17 SPDIFO#0 SPDIF Output Signal
18 GND Ground
19 SPDIFO#1 SPDIF Output Signal
20 GND Ground
21 GND Ground
22 SPDIFO#2 SPDIF Output Signal
23 GPI0 Digital Input
24 GPI1 Digital Input
25 OUTMIDI Midi Output
26 GND Ground
27 INMIDI Mini Input
28 GND Ground
29 KEY
30 KEY
31 ADCSDO2 125 Audio Data Input
32 GND Ground
33 ADCSDO1 125 Audio Data Input
34 GND Ground
35 ADCSDO0 125 Audio Data Input
36 GND Ground
37 I2SCLK 125 Serial Bit Clock
38 GND Ground
39 I2SFS Frame Sync
40 GND Ground

Can I use the 4-pin CD/DVD audio-in (LGGR) as an analog input from another source?

Short answer is yes.

I made a 4-pin CD/DVD audio to female 3.5 mm cable. Then, I connected analog audio into that connection and it worked. Woot. Now, I have 2 analog inputs.