Acoustic impedance is a ratio of pressure to flow. For instance, we measure it at the embouchure hole of the flute because it tells us a lot about the way the air jet from the player's lips will interact with the instrument itself. So it tells us about the acoustical performance of the instrument, in an objective way that it independent of who might play it, and it allows us to compare subtle differences between instruments.
Electrical resistance is the ratio of the voltage applied to the electrical current it produces.
AC is more interesting because the impedance can vary with the frequency of oscillation of the current. Flow is measured in cubic metres per second. Finally, sound pressures have a large range. For this and other, psychophysical reasons we use logarithmic scales for sound level and impedance. For a linear quantity like sound pressure or impedance, you can convert to dB by taking the log of any ratio, then multiplying by 20.