As the previous posts indicate, it's a tough question to answer. It really depends on the
level of the production too. I suspect the question might be loaded, too, from the OP.
I've always relied on the director's guidance, which sometimes as a sound person I receive from the MD anyways. If it's things specifically related to the pit mix or vocal mix, probably the MD. If it's not musical content, and we are talking about SFX vs spoken word or music, then probably the director.
In my experience, the SD can try to make those decisions until someone higher up the food chain decides they don't like whatever the SD was doing. Really depends on the relationship between parties involved and the
level of the production. Anything at or below college
level, probably the SD. Anything above that or getting into semi-professional paid gigs, the Director or their designee.