Reduce size of sound clips

I have a folder with about 600 short (2 to 5 seconds) .wav sound clips.
Total size is 225MB

I need this to be reduced drastically.
Each one to be reduced as much as possible, but still being audible/useable.
So each to be converted from stereo to mono, bit rate reduced, file type changed (not sure which is best to reduce size?)

Looking for sound engineer expert type with the right software.

Thanks