Yes, it actually is sol: it's traditionally written that way when the tonic notes are spelled out, and often referred to colloquially as the Solfa or Solfège scale. That final L is hard to hear
The Puzzle: RE + MI = FA. DO + SI = MI. LA + SI = SOL. (Replace letters with digits and have the sums be true.)
The syllables for major are DO, RE, MI, FA, SO, LA , TI, DO. (That's pronounced doh, ray, mee, fa, so, la, tee, doh.) Each syllable corresponds to a scale degree. The 1st degree of the scale is DO, the 2nd is RE, the 3rd is MI, etc. Here's a chart for the syllables: The seven syllables normally used for this practice in English-speaking countries are: do, re, mi, fa, sol, la, and ti (with sharpened notes of di, ri, fi, si, li and flattened notes of te, le, se, me, ra ). The system for other Western countries is similar, though si is often used as the final syllable rather than ti .