Hello!
I’m an engineer student working on a group project where we have a BlueROV2 to work with. Our task is to attach a hydrophone via a sound card to the ROV and use the hydrophone to locate and travel to a sound source. We plan on connecting the sound card to the Rpi with a USB-C cable.
The thing we are interested in knowing is how we implement our own code to accomplish this. We want to know how we get to the source code and how we can upload what we have developed.
Hi @CarlS, welcome to the forum! 
The BlueOS source code is available on GitHub, and there is development documentation available for understanding the different parts of the codebase (feel free to ask questions if something isn’t clear).
Given you’re considering adding support for an external peripheral you’ll most likely want to do that via a BlueOS Extension, although how much of the desired functionality needs to be handled by the extension is dependent on whether you want some of it implemented on a control station computer at the surface, and whether BlueOS ends up with built in audio support within your project timeline (@joaoantoniocardoso will be working on that in the not too distant future, but there are some other features that are being prioritised first).