Valero Air flies 4 seater aircraft (or bigger ones according to demand) twice per week, but this is second hand info. They have a facebook page with schedule.
Looks like they only fly Sat and Monday......Do not buy round trip, in advance, might get stuck, holding return ticket, with questionable return flight.....
And you can always catch the Metro out of Sosua and do a lay-over in Santo Dom and catch the bus from there that goes to the airport and area, first thing in the morning......... Always works for me and very reasonable priced :bunny: