Booming Voice makes range 6 CEs unlimited range. This means that while the commander and the affected character can be any distance apart, you must still be able to count distance between them.
If one is locked in a room, you cannot count the range between the two and thus the CE would not affect the other character.

Both of the above are correct, and here's even more clarification, just to make sure that all the bases are covered:

The "Booming Voice" ability itself DOES work through walls/closed doors. Mas Amedda can "locked" be in a room alone, and you can have Thrawn outside the room, with no path between Mas and Thrawn, and Thrawn still benefits from Booming Voice. It's the CE with the extended range that does not work through walls/doors. Booming Voice is a board-wide effect (like GGDAC or Rieekan's CEs), so Mas can be anywhere and every commander benefits from Booming Voice no matter what.

The best way that I've found to think about Booming Voice is to say that it makes every CE with a range of 6 now have a range of, say, 500. It doesn't become board-wide (like GGDAC, Rieekan, or Booming Voice itself), so you still have to count from the commander to the target around walls, but it as long as there is a path (no matter how long that path is) the CE extends to the intended recipient.