A question about AC Ground and Shielding
You seem to be lucky. Many power adapters have protective earth pin of the mains plug in use and it's directly connected to the DC output. The result: horrible ground loop hum if it's a part of an audio system which uses non-balanced signals. The phenomena really stretches the nerves of people who try to use a PC as a part of a bigger audio system.

Your audio equipment probably has an audio input. Connect your metal case to the signal ground of that input and do not allow any other circuit points in your equipment make any electric contact with the case.

Shielded internal cables: If a shield is not a signal path, it can well be connected to the local ground of the carried signal. Let the other end of the shield be free to avoid local ground loops. If the shield carries signal, it shouldn't be connected to anything else. if you have several uninsulated shields, which do not carry signals, your best bet is to insulate them and connect to the groundpoint of the carried signal. If you cannot insulate them and they make contact with the case, connect them firmly to the case to avoid microphonicity.

If possible, do not allow the case make any contact with concrete floor or other audio equipment.