That was a long reply? I would advise against going through my post history then.
The difficulty of the fix is relative to what you have, what you know and how good you are at this sort of thing.
If it turns out the speaker is just off its mount or something and fixing involves unscrewing the screen, popping it back on the locator pins and putting it back together then I guess that would be easier. If it turns out someone really broke something inside there then you are going to have to align it manually, tape it/glue it down and get it done that way.
It if turns out the magnetic sensor/switch is broken then yeah that would be annoying to try to sort, and it might not even be the sensor as much as some other component or trace in the chain that is acting up. I have not heard of this happening to a DSi before, but at the same time it is not as immediate and deal breaking as broken screen, not charging, volume issues, power slider or normal game/shoulder buttons not working which we see often enough -- within the DS/DSi system it is actually counted as another button so some games do use it but most just trigger sleep mode when it gets activated, which for most is just a minor convenience and around here flash carts were popular and don't actually drop their power consumption much in sleep mode. Likewise I have seen enough industrial and automotive magnetic sensors fail and those things are built to handle far more abuse than the DSi ever was.
I feel bad at times telling people to just get a whole new board/device but where I would bother to go through a £1000 laptop, a £[if sir has to ask] industrial machine that is costing untold money being down or just a really rare device I take a look at the DSi and there are a thousand for sale right now probably for less than it would cost for a few hours in the electronics shop or even to think about gearing up to make an honest go at fixing it.