Hi Blueskill :wave:
If the speaker 'impedance' (Ohms) is higher, the volume is proportionately lower (more resistance), if the speaker impedance is lower, the volume is louder.
It's important to match the impedance correctly though, a lot of transistor/IC amps don't like lower impedances and tend to 'short out' (tech. term for fry :grin

, too high and the sound quality suffers, but they don't fry.