I have a pc usb and front panel audio circuit board, and I only plan on using the audio connectors. Can I cut off the USB ports part of the board and still have functional audio ports? The usb does not connect to the audio connectors on the board.
General rule of thumb, no. Some PCBs have hidden inner layers which can hide circuitry. Even two layer boards have highly interconnected traces.
In your case though, the board is nothing more than wire to connector. There is seemingly no hidden gotchas. So the answer is most likely you will have no issues.
Most likely. You should try to maintain the same grounding (looks like it takes place through the mounting holes as well as the black wire). The two mounting holes are both connected together, so one will suffice for electrical grounding purposes.
Arrows point to places where the ground plane is connected off the board.