Any audio/ Mp3 editing software will be of use. Suggest to try FlexiMusic Wave Editor, using Save As command save the Mp3 file in the software. While using this command, it will ask for change in bit rate where you can change (lower) the bit rate of Mp3 files.
Play the wav file in any audio editing/ converting software, suggests FlexiMusic Wave Editor. Using the "Save As" command, convert and save the wav file as Mp3. To save the file as Mp3, need to install a separate command line Mp3 encoder/ decoder. Lame win 32 will be of use.