Features for audio levels imbalance in films between speeh/music/effects
I would try a sum and difference calculation between the L and R channels to start. Usually when film audio is mixed, the speech is in both channels in phase while the special effects is out of phase.

So if you do:

X = L+R;
Y = L-R;


X should give you the signal with more speech and Y should give you the other effects. Then if you do an RMS calculation on X and Y, you can find how much are they off by.