Fixed in r19440. Note that the error was not in the saving code, but in the loading code, so the exported files are fine and should work in other applications. There was an unrelated issue that I also fixed with saving, though: 16-bit mono files were saved as little-endian rather than big-endian. It went unnoticed until now because OpenMPT can already heuristically detect this situation because other software (FT2, Awave) did the same.