Nice work!
Are you using the Votrax core from MAME or PinMAME? MAME's will be the latest and had some fixes applied in the past 6 months.

To play some speech in Black Holes sound test mode, see the flowchart from the manual here:
https://pinside.com/pinball/forum/topic/black-hole-no-sound/page/2#post-5190666

From memory, Black Hole uses the voice pitch change when you tilt the game and it repeats the word "Tilt" a few times each with decreasing pitch.

I think most Black Hole owners would like to have the pitch change support. Of course it depends if you're able to implement it.

To get an idea of the the pitch change effect, try it in PinMAME.
> pinmame blckhole
Enable speech in the sound board DIP switches via the 'TAB' key menu.
Press F3 to reboot.
After 5 seconds game boot-up, insert a coin by pressing the '5' key.
Then press the '1' key to start a game.
Pressing the 'Insert' key is the tilt switch and you will get some idea of the changing pitch effect.

I'm not sure but the pitch change might also be used on the voice effect when the ball enters the lower playfield. And also the slam switch effect ('Home' key in PinMAME).

The 5 signals should be used in binary fashion. For Black Hole, these are the commands interfaced to the sound board for speech that I see running PinMAME:

0x1F = wait for new command
0x1C = "Tilt Tilt Tilt Tilt Tilt Tilt Tilt Tilt Tilt Tilt" (descending pitch)
0x12 = enter lower playfield "Ooowww"
0x10 = slam "Ooowww"
0x08 = "Shoot Captive Hole"
0x07 = "Complete Bank For Re-Entry"
0x06 = "Enter Gravity Tunnel"
0x05 = "Re-Entry Attept Has Failed"
0x04 = "Re-Entry Accomplished"
0x03 = "Extra Ball Lit"
0x02 = "Shoot for Special"
0x01 = "G-Force Accelerated"
0x00 = "Ball Captured"

Items in "quotes" are voice word/effects.

0x1F (all high on signals S16, S8, S4, S2, S1 to the sound board) tells the sound board to be ready for a future command.

I tried Volcano, it uses almost the same pitch change effect with the "Tilt" word as Black Hole. Most of the Ooowww voices are different pitch from another.

0x1F = wait for new command
0x1C = "Tilt Tilt Tilt Tilt Tilt Tilt Tilt Tilt Tilt" (descending pitch)
0x1B = "Volcano"
0x18 = "Eeewwww"
0x15 = "Shoot Lit Volcano"
0x14 = "Eruption"
0x13 = "Oowwww"
0x12 = "Oowwww"
0x10 = slam "Oooowww"
0x0D = "Oooowww"
0x0B = "Ooowww Ooowww"
0x08 = "Danger"
0x07 = "Lava Rising"
0x06 = "Ball Save"
0x05 = "Fire Pit"
0x04 = "It's Gonna Blow"
0x02 = "It's Gonna Blow"
0x01 = "Oooowww"
0x00 = "Throw Me A Sacrifice"

Thanks, I inverted the five command bits in the post above with relation to what's on the five S16-S1 sound signals to the sound board, and corrected the spoken words for Volcano. The voice command originally listed as 0x18 is now 0x07, i.e. sound signals S16 and S8 go low.

Scott Savage in that video has a pinside account SMSavage32 and posted about taking on this project above if you scroll up to post #75
He hasn't been here for 8 months but hopefully he'll return once it's complete with details.