There are 2 different sound "boards" in this game. The one on the MPU and the seperate sound board. Your problem might be on either "board".

The sound board on the MPU has 2 outputs and can generate sounds and speech.
The seperate sound board has up to 4 outputs and can generate sounds and speech and has 2 FM outputs for music.

You can narrow down which of the 6 sound lines is low by disabling one of the sound boards

Pull the processor from the remote sound board and this will only let the two MPU based ouputs works. Put that back in and then pull the processor from the MPU board (not the main processor, but the one nearest the sound roms) and try it again to test the 4 from the seperate sound board.

This will enable you to isolate the problem to a specific board.

Once you have isloated the problem to a board then you can start trying to diagnose it more accurately.
There are some op amps (pre-amps) for each sound line as well as a couple of in line caps that are the likely culprits for the low sound.

Popping the Banzai eproms into the Dr Dude board and putting that in Banzai will also accomplish almost the same thing.

Remember the system 3-7 sound board? Starting with system 9 that sound board is on the mpu. And the system 6-9 speech board got moved to the mpu board starting with system 11. They were both removed on system 11C.

Those caps are not for the sound. Those are for the power in or the reset section of the board.

Cap C8 is inline with the MPU generated sound before it gets mixed in with the MPU generated speech.

Op amp at U4 is inline with the speech as are caps C9 and C15.

The Op amp at U5 is after the speech and sounds are mixed.

Do you know if the MPU based speech is working? I would remove the processor on the remote sound board to disable sounds from there and see if the MPU based speech is working or low.

Check out Page 3 of the system 11 MPU schematics and look at the lower left corner to see the sound generating chips. The CVSD at U3 generates the speech signal the the DAC at U2 generates the sound effects.

I would start with the cap at C8 and after that try the 1408 DAC at U2.

Ed at Great Plains Electronics has a replacement for the 1408

DAC0808LCN

Isn't it an electrolytic cap at C8?

https://www.greatplainselectronics.com/proddetail.asp?prod=CEA-10uF-50V-VX

You can always increase the voltage value on caps, as long as it still physically fits in that location.

I suppose you can change it to a tantalum.