How did you measure 91dB? If you can measure it again you can adjust it down by the dB difference by entering an negative -dB value into the Amplify effect. …If it’s now 100dB, simply reduce it by 9dB (Amplify by -9dB) and you’re done!
the over all sound track is rendered much louder than the 91db I started with!
That’s normal. It happens if your compressor adds make-up gain. That’s how compression is most-often used… Compression typically “pushes down” the loudest parts and then you can bring-up the overall loudness without clipping the peaks. If your chosen compressor allows you to turn-off make-up gain, compression will bring the average down a little, and the peaks &loud parts down more.
91dB is a probably a ReplayGain SPL number (acoustic loudness). There is a reason for using an SPL level. When the equal loudness curves are taken into account, you need to know the listening level if you want to match loudness. There is a [u]ReplayGain plug-in[/u] for Audacity, but I think it uses a digital reference instead of an acoustic SLP reference.
The digital dB reference of 0dBFS (zero decibels full scale) is the “digital maximum”, so digital dB levels are normally negative. The acoustic SPL reference is approximately the quietest sound humans (with normal hearing) can hear. So, SPL levels are normally positive.
SPL levels and digital levels are not calibrated because we all have a volume control and some speakers & amplifiers are louder than others and it depends on how far you are from the speakers, etc. But, there is a correlation - If you reduce the digital level by 3dB, the acoustic level will also drop by 3dB.
And, there is a standard ReplayGain calibration/calculation (as long as the equal loudness curves are taken into account) so it’s a “simple subtraction” but I don’t know what the calibration factor is… The 87dB ReplayGain standard is somewhere around -20dB digital. (That’s NOT a peak level, it’s a frequency-weighted RMS level.) But as above, you can’t know the true SPL level from the digital level unless your system is calibrated.