Any meaning to the magnitude of a FFT when audio is recorded using a PC?
Actually, to say the either the sound values or the DFT magnitutes are unitless (or meaningless) is quite misleading.  The sound values actually represent changes in the sound pressure level at the microphone.  This pressure has physical units.  You can think of the microphone and A/D conversion as a "transducer" which converts the units into something else.  Ideally, this conversion is linear.  So the units in the sound file do have meaning, but most of the time it is not worthwhile to calibrate them to the actual physical units.

The DFT magnitudes are then in the same units, after you normalize by 1/N, where N is the sample count of your DFT.  The bit depth has to do with precision, not scale.

Your conclusion is still correct, but now better founded.  You can compare two signals on the resultant scale.

Ced