I'm sorry that this is how your experience has gone thus far. Given that you wrote up the problem, we really should have sorted this out on the first go round for you. The inverter was probably a reasonable part to replace, the speaker - I'm not sure given the other symptoms. The next step is most likely the system board as the audio is integrated, as others have suggested.
I'm not clear if you have sent it back in to the depot again (if so, could you send me the case number?).
If not, and you still have the system and would like to pursue some other options, I can help with that as well - it will be best if you can send me a PM with your contact information (address, phone) and the MT and Serial from the bottom of the unit.