Hello,
Yes the solenoid block for that transmission is internal, but you may find damaged wiring of connectors leading to, or internal to the transmission. Note, the code does not say the part is bad, but tells you the circuit has a detected issue. Time to test.
The car audio system causing a misfire...not likely. Also, a misfire code would read (p)0300, 0301, 0302, 0303, 0304, 0305, 0306, 0307, or 0308.
Audio installations on modern vehicles should only be trusted to someone who is certified to service OBD-II and CAN-BUS systems.
The electronic systems in these vehicles are much more complex than they were in older models.
Look around and see which audio place folks use for their new Mercedes Benz and go there.
Your vehicle is more Mercedes Benz than you may think.
For the transmission, use the link below to find a certified shop you prefer. They will have professional scanning equipment which can help them pinpoint the issue, or at least know what to test first.
http://repairpal.com/jeep-within-10-miles-of-atlanta-georgia-30310
