Let's assume that you have an external source of
mechanical power, and are using it to move hydraulic
fluid from a low pressure reservoir to a high pressure
reservoir.
Now, using a high efficiency linear actuator
(piezoelectric or a solenoid, most likely), open and
close a valve at 60 Hz
(or 50 Hz for you Brits). This
valve leads from the high pressure reservoir, to a
specially shaped reflector with an integrated linear
alternator, to the low pressure reservoir.
The opening and closing of the valve will produce a
sound wave, which bounces off of the linear
alternator, generating electrical waves at the same
frequency as the sound wave.
The speed of sound in the hydraulic fluid, and the
distance between the valve and the alternator, and
the frequency (60 Hz or 50 Hz), will determine the
phase difference between the power supplied to the
actuator and the power produced by the alternator.
It may be desirable to have a variable-length section
(such as a trombone has) between valve and
alternator, to control whether the system produces
real power, reactive power, or some combination of
the two.
If it is not necessary for the system to match it's
output phase with that of an external power source,
then we can replace the electrical actuator that
drives the valve with a acoustically driven actuator,
using appropriately timed feedback from the output.
PS: This idea was inspired by the auxetophone, which
is surely the coolest type of sound amplifier there is :)