Producers of the Broadway revival of “Porgy and Bess” touted the return of topliner Audra McDonald to the show on Thursday night following a two-week absence that has since hindered sales since it began on June 20.Last month Rialto fave McDonald scored her fifth Tony for her perf in one of the Gershwin musical’s title roles. But soon thereafter, the thesp — a draw for many avid theatergoers even before she nabbed the kudo, alongside the production’s win for musical revival — exited the production for a period of doctor-mandated vocal rest. Her sick leave coincided with a notable dip in the production’s B.O. Whereas the show took in $655,000 in the week prior to her absence and played to houses at 75% of capacity, the most recent frame saw receipts drop to about $460,000 from an average attendance of just 48%. Things seem poised to perk up, however, as McDonald rejoins co-stars Norm Lewis and David Alan Grier. Show’s success on the Rialto, where it’s grossed about $19 million since beginning perfs in December, had already prompted producers to extend the production’s limited run from an initial closing date of June 24 through to Sept. 30.