Ah, all becomes clear. Yes, you're making a transformer, a transformer designed to do impedance transformation. The impedance of free space (i.e. the 'medium' for radio waves) is 377

. The impedance ratio for a transformer is
Z
1/Z
2 = (N
1/N
2)
2Where the Zs are the impedances seen by the windings and the Ns the number of turns.
So your 90:25 turns ratio becomes a ~13:1 impedance ratio. Your 377

free space impedance now 'looks like' ~4900

to the rest of your circuit. Your signal voltage goes up, and your signal current comes down. A piezo earpiece is quite happy to be driven from a high source impedance so you're getting a voltage gain of ~13 for 'free' while losing drive current that the earpiece wouldn't have taken advantage of anyway. That's a simplistic explanation ignoring the fact that we're dealing with a tuned circuit and complex impedance, not scalar impedance, but it's enough to get a flavour of what's going on.
Why your transformer will behave happily at radio frequencies while being nearly useless at audio frequencies requires a longer explanation than I'm in the mood for typing, especially as a proper explanation can't avoid typesetting (or trying to typeset) some maths, and it's also past my bedtime. Succinctly, and completely trivializing it, it's to do with the rate of change of magnetic flux which will naturally be faster for higher frequencies. One for the textbooks really - anyone got a good online resource that would explain this?