I believe this is an Indian name. Several sites say it means 'gold' or 'golden', but I'm not totaly sure if these sites are reliable.
http://home.casema.nl/rastarebel/Meisjesnamen%20S.htmhttp://www.mitaliperkins.com/Mitali_Perkins_Bio.pdfhttp://www.indiaparenting.com/names/names1.cgi?param1=girl¶m3=sss¶m2=
Sonali looks a lot like the name Sonal, that according to this site means 'golden' as well.