According to https://darksouls.fandom.com/wiki/Executioner_Smough
In the code for the Japanese version of the game, Smough's name is displayed as '処刑者スモウ', which translates to read as 'Executioner Smō'.
Without going too much into detail with Japanese, based on the spelling: "スモウ" (Su mo u), his name was originally intended to be read as one of the following options:
- Sumō (possibly)
- Smō (most likely)
- Sumo-oo (least likely)
- Smo-oo (not likely)
The "u" that follows a consonant ("s" in this case) is usually short, to the point of being inaudible.
The "u" that follows an "o" usually combines with the "o" to make it sound longer (written as "ō"). In rare cases, you can hear a very faint "oo" (as in "loop") after the "o" sound.
As a result, the most likely candidate sounds like "small" but without the "ll", and with a long "o" sound at the end.