I was subpoenaed for a jury trial by both plaintiff and defendant and given a check by each party. That jury trial was postponed. Then I was subpoenaed again by the plaintiff only. The case has been settled. Neither side has asked for their checks back. Do I need to return the checks or do I cash them?

Did you in fact lose time from work or incur other expenses? If not then you should return the check.

You lost some amount of time just concerning yourself with plans and communication.
I'd cash them and not spend the money for 6 months.
They may want just some back. If it's small, maybe nothing.
Are you sure you didn't sign a contract, anything, that would spell this out?

Typically, the rules require that with service of a subpena, the wtness be paid a witness fee. It could be cash, but for convenience you were served with a check and you could have chashed it. At the time you were served with the subpoena and check, you were obligated to appear. When they cancelled the trial, your obligation to appear went away.

The money is yours.

Joy, there would not be a contract; it's a matter of court rule.

In NY the person subpoenaed keeps the money - if he appears or doesn't appear for any reason other than refusal to appear.