If they're 0.25w speakers and their impedance is 4 ohms, then you don't need 12v to drive them. The circuit you gave could damage the speaker when the mosfet turns on.
Its a small speaker so the current spikes on turn-on could be too large - reducing the 220uF blocking capacitor to a smaller value is wise (perhaps try as little as 1uF at first, then if its not struggling up the value until its loud enough).
I suspect this circuit as is would drive a much larger speaker fairly well.
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