To be honest, the use of 'passband' in EN is questionable; unless there is some specific reason given elsewhere in your surrounding context, there is no real justification for qualifying it as 'passband' — the 'out-of-band' voltage sensitivity would be largely irrelevant in the context given here. Sound to me a bit like techno marketing speak — though it might be helpful to know just WHAT it is lower or higher than?

The meaning is, however, clear enough: the voltage sensitivity (usually the only parameter of interest in this sort of context, since current sensitivity is usually irrelevant for even quite medium impedance systems) across the passband (= frequency response)

In other words, they are talking about the in-band sensitivity — the only useful parameter here, though the out-of-band sensitivity is unlikely to be a very meaningful criterion.

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Note added at 2 heures (2015-10-20 17:13:20 GMT)
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I think what they're seeking to say is that "Many personal digital music players have a limited output..." but ours have more than enough output to spare.

The point is, higher-impedance headphones will require a greater output voltage to drive them (higher impedance will mean less current, and since Power = I [current] × V [voltage], the higher the impedance, the higher the voltage that will be needed to produce the same (electrical) power.

Conversely, if the headphones have lower sensitivity (and voltage sensitivity is simply the most usual parameter quoted) , they will require a higher (electrical) power input in order to produce the same (mechanical = sound) power output.