People look for a Milos to Lavrio ferry for good reasons. Lavrio, on the south-east tip of Attica, is about 35 km from Athens International Airport, and for some travellers it is closer to where they are staying than Piraeus. There is also a Lavrio to Milos sailing in the other direction, so it is natural to expect a return.
The honest answer is that this direction is, at best, irregular. Our own older route data showed Blue Star Ferries sailing it in the warmer months, but Ferryhopper, which lists both directions on its Lavrio to Milos page, had no Milos to Lavrio sailings in any month of last year's data and showed no price when we checked. The outbound Lavrio to Milos boat is itself a once-a-week, multi-stop voyage that usually takes around 16.5 hours, so even if a return appears on the booking page for your date, expect a long, indirect trip.
The good news is that there are two reliable ways to get from Milos to the Athens area. The first is the daily ferry to Piraeus, the main port of Athens. The second, between May and September, is to take a short boat from Milos to Sifnos or Serifos and then the Magic Sea Ferries high-speed service from there to Lavrio, which Ferryhopper lists on 4 to 7 days a week in season.
Before you give up on the direct option, search your exact date on the booking page: schedules change from year to year, and if a Milos to Lavrio sailing is on sale, it will show there. Just do not build a flight connection around a boat you have not been able to book.