Did your Audya have OS3 from the factory or did the shop install it? You need to ask the shop because they may have done it wrong (but might be reluctant to tell you). If it were my Audya I would reload OS2a, styles, OS2C and then OS3 like it says on the UK website. If it had OS3 installed from the factory you don't need to do that. Your Audya should just work, if it doesn't work there is obviously a problem and I don't think any of us can help you with it..

You also say your computer doesn't see the drivers, but then say the Audya is playing MIDI files from your computer but in a strange way?

You really need to get on to the shop you brought it from. You could be wasting hours for no reason.

Also, you shouldn't be using the USB ports on the front of your Audya! they are for devices like memory sticks etc. Like I said before you need to use the rear USB socket.