Have Managed Plugin/DLL Reference Source Code

The studio I'm working with are converting their code to DLL's but I've just added FMOD to the project.

The code in the DLL's can't access FMOD so we've tried adding the FMOD code to the VS solution. This has caused a lot of errors and, while I'm still trying to solve it, is there any way to get the code in the DLL's to have access to the source code? Primarily the runtime code for FMOD?

Although I'm not completely 100% on how all this work, there must be a way to reference the source code so this can work?

The only thing I can think of that would not let this work is if the build order for Unity means, even if FMOD is built first, the DLL's from VS are already built and will never be able to reference the source code. But I haven't got anywhere when looking for this specific issue. The most I've got is how you can reference the Unity DLL's, but I would like to reference an FMOD Mono Behaviour, not FMOD's DLL's that have code that isn't really applicable to me.

If there is any way this can be done then please say so as this is becoming the bane of me.