Thanks for all of the replies!
What I have is a standard mono "mini-jack", which has no contacts. I was wondering if I could make something up just with that. I have a basic knowledge of electronics and can't find a good solution.
Thinking it out, I understand that if there's no external device connected, the signal will just skip that part of the circuit and go to the speaker, however I'm not that sure it will happen the other way when connecting something to the mini jack port.