Wow, sounds like it'll be fun, fun, fun to drive. BMW make, and made the best straight sixes in the business. Not only should your engine be smooth as silk and sound fabulous, but it should also have some serious welly.
Be careful in this car! If your driving experience is limited you absolutely must make sure that you go into corners slowly enough, and you don't floor the throttle with the steering wheel at any sort of angle. There will be turbo lag. When the power comes in it will all be at once, and transmitted to the rear wheels. These will then spin, in all likelihood.
I feel I ought to advise you to sell it and get something less, um, vivacious. It's a BM, however; small, fun and built like a tank.
Have fun and please be careful!
For the left speaker, start by checking the wire connection at the back of the stereo unit itself, before proceeding to check the connection on the speaker cone which should be unscrewable from behind the grill.
www.pistonheads.co.uk is a great site with a separate BMW section and lots of chaps who are an awful lot more knowledgeable than I.
Also try
www.bmwboard.com and
www.bimmerforums.com