The Wikipedia article on Korean phonology says ᄉ can be voiced just like ᄀ, ᄃ, ᄌ and ᄇ:
[The] characteristics [of /s/] are nearest to those of plain stops, as it generally undergoes intervocalic voicing word-medially.
However, I don’t remember ᄉ being mentioned in voicing contexts in most materials I’ve read. Have I just been blatantly missing it, or is the situation of ᄉ not that straightforward?
(I could say I don’t really notice it being voiced when listening to e.g. ‘의사’, but neither am I experienced with spoken Korean nor do I actually trust my ears in situations like this. At all.)
PS: I’ve just noticed that it isn’t given as voiced either in Wiktionary entries, e.g. ‘의사’.