That "interface" you are talking about is simply wires running from the front panel to your soundcard, or your motherboard's sound input/output circuits. It can still cause damage as most guitars (and even more so basses) output hi-gain signals. To hear it properly through that socket, you need to have a pre-amp that outputs a low-gain signal. I used that setup years ago before USB devices were available for guitars, and when proper audio interfaces (at least for PCs) were prohibitively expensive.