Firstly, congrats on picking up a DK! Would love to see some pics of the cab in general.
Now to your issue. It's not uncommon for the speaker to produce a hum when no game audio is being produced. Usually a cap kit on the audio board can help with the buzz/hum.
However, are you saying that even when game audio is supposed to be playing, there is only a hum and no game audio?
Some things to check:
audio volume pot on the remote board - make sure it didn't accidentally get turned down, so that all you are hearing is the native hum
Follow the cable back from the speaker to the audio board. Make sure everything is connected up as it should be. Also beware, if you find more than one thing unplugged at the audio board, there are two connectors that are nearly identical and can be hooked incorrectly. If they are hooked up incorrectly, bad things happen (your audio board will fry).
Check all power cables from the power supply to the game board, and make sure they are all connected. If the game PCB is a 4-board set, going from memory, it requires a dedicated power connection from the power supply to the sound ("SOU") board, which the 2-board set does not. Someone correct if I'm wrong on this.