Assuming they all open up in Audacity, you can use Effect > Amplify on each one before you mix them together.
That’s the quick and dirty version. That doesn’t process loudness, but it does make the blue waves line up more or less, and that’s usually enough.
If you like to change the character of your show – like bopping back and forth between music and talking, then Chris’ Compressor is pretty much the only automated tool left.
http://pdf23ds.net/software/dynamic-compressor/
That one does process loudness, but it has the same effect as running your show through FM radio station processing… everything comes out flat and the same volume. Try it.
Koz