A rolled up newspaper tube held against your ear and in front of a suspect driver can help. To be sure, though, a soldering iron to disconnect the leads and an ohm meter will tell you more. (The piezo the exception here as it's a capacitive load, not resistive.) If it still tests alright, but you don't hear anything, then connected to an amp at low level with a sig gen feeding it (free apps are available for smart phones) can help you test for one that might be binding in some odd way.
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Which driver are you suspecting?
