I would use basically the original arrangement, but put the sense resistor in series with the mosfet source connection instead so that you measure the mosfet source current. Reduce the resolution of the ADC to 8 bits so that you can do faster conversions. Monitor the source current while the mosfet is turned on, then when it reaches the value you want, turn the mosfet off for a predetermined time, say 100us. The snag with this is that the switching frequency will be be the audible range.
Alternatively, use a diode and capacitor to turn your current sense op amp circuit into a peak or average current detector, then you can modify the PWM amount according to the current, and use a supersonic switching frequency.