Need to down size MP3 to lower bit rate

I have a massive audio book collection converted at 128kbps and this is still too large: Is there a program that can re-sample to a lower rate of 64kbps or even hopefully 32kbps? I can't re-rip, because I no longer have the CDs. This must be a MP3 to MP3 exchange.

I had originally used WMP11 and 128 is the lowest, I tried Audacity but I was unhappy with the sampling (there was a faint and annoying "pop" every eight seconds or so).

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