Can a wire nut be in wall without a junction box?
I'm not an electrician, but I would say no, if you want to keep things to code.
*Best way is to redo the wall plate at site.
(This is easy if cable is running from below, but if cable is running from above, you have your extension problem.)
Downside is that you will need to cut into the wall and install new boxes.
*Do it external and keep the wall plate where it is and come up with a creative solution to hide the wiring such as a cable racetrack or piece of trim. (Can paint a piece of trim to match the wall, maybe not ideal, but an easy solution.)
*Just leave cabling exposed.
(Definitely not the prettiest solution.)
*Don't move the speaker location but just make certain your mount has the range of motion to aim at your LP.
(Any good mount should give you left to right pivot as well as 15-20º downward tilt.)