Double check your connections; the only options are a) it's hooked up wrong or b) your game is not able to provide enough power to both the LEDOCD and the sound board

It is possible that a hum could be caused by the switching of the LED OCD. It does switch at a higher frequency, and occasionally, this can couple into the audio circuits. Moving the board and/or wires could help if this is the case. The garbled speech, however, is a different issue.

Unless it is installed incorrectly, it would be next to impossible for it to make your plunger weak. It pulls power from the same power source as the original lamps, and a game full of LEDs plus a LED OCD would pull less power than a game full of incandescent bulbs. It doesn't pull any power from other sources, so it's not going to starve any other circuits. It also doesn't connect to any of the digital logic circuits, so it won't affect any of the CPU, sound, or coil operation.

If it continues, I would suggest disconnecting the LED OCD just to eliminate it, but I am confident that it is not the cause of the garbled speech or weak coils unless it is incorrectly installed.