"Biletele noastre sunt dus-întors."

Translation: Our tickets are round trip.

[Pop60]

"Our tickets are returns" would be natural English and I think a correct translation.
Strangely I think "Our tickets are round trip" could be OK but would probably be used about more exotic journeys - transatlantic sea crossings, special air journeys etc, I do agree not typical at the train station!

[WarsawWill]

I think this is a BrE vs AmE thing, While we Brits say "return", I think Americans say "round trip". And these courses are written in American English. However, "returns" should also be accepted. Reported.

[Coriander18]

Our tickets are returns (short for return ticketS).

[JsBangs]

The audio REALLY sounds like she says "de dus-intors" instead of just "dus-intors".

[JonEnMiljon]

It's the good old ”suntă” glitch you're hearing ;)

[RogerPlant2]

Your suggested translation is not good, and would not normally be used in English. You can either say "our tickets are for a round trip" or "we have return tickets".