OneByte - 128
changes the range 0 to 255 (unsigned range) to -128 to 127 (0 - 128 = -128, 255 - 128 = 127).

That will give you a signed (+/-) wave sample. If you wanted to treat it as a 16 bit signed sample you could simply now multiply by 256 to put it in the range -32768 to 32767.