Hi, not sure about the small plug (if you mean the small coax connector? Looks like an SMA or SMC but can't really remember). I'm thinking that it is a lead that brings up some kind of signal related to the speed of the car that could be used for increasing the volume of the system and/or the feature for the satnav to continue working when it loses its GPS signal (by calculating the speed of the car, etc...). The other connectors you can see in the pic earlier in the post are power, speakers, remote control and antenna.
The sound quality is very good, I have only used MP3 discs (lower compressed quality), USB (MP3 again) and the radio and am very impressed with the audio quality, it does take some setting up (audio settings, driving position, parametric EQ) and have to admit that I havent done this properly yet, but I cant fault it at all, and being an ex-audio engineer I am quite critical! In fairness, the Audio10 wasnt bad, but this unit, in my opinion is better than the Audio10 CD is (judging it on audio quality only, not features). Obviously, in features, the unit is very, very good and I highly recommend it.
My ONLY bad point, if I had to really try to fault it (so far) is that the display looks a bit 'plastic' and reflects the light which 'sometimes' makes it hard to read, but i'm being really picky here!
Any other questions, just ask!