FYI there are 3 methods of inverting samples that I have discovered so far.
Method 1 (8-bit or 16-bit) - Setting volume of sample to -100.00 in the Instrument Editor (works for both 8-bit and 16-bit samples). The traditional way, I guess...and definitely the most straightforward one out of all.
Method 2 (8-bit only) - Setting sample data to 16-bit, followed by "Swap byte order", "Change sign" and "Swap byte order" again. Optionally, revert the sample data back to 8-bit. This is the method I knew first, albeit tedious. You can put in 16-bit samples, unless downgrading its sound quality is not your intention.
Method 3 (8-bit or 16-bit) - Click on "Change sign" and put in "0" for "Ignore bits from MSB" (MT v1.01.00 and after). As simple as Method 1? Maybe, but definitely a lot simpler than Method 2.
Feel free to expand the methods available