Look for an audio editing program, like audacity. Simply export it as a lower sample rate, like 96 kb/sec, instead of something like 256 kb/sec. Sound quality shouldn't be too bad.
Audacity does require you to download an extension called 'lame', in order to export .mp3 files, however.
You can get lame here. Just put the "lame_enc.dll" file inside the Audacity folder (you don't need to install it lame, or do anything else with the contents of the compressed folder). Then, when the program asks where it is, browse for the file.
Or, if you're lazy, you can send me the file, and I can do it for you (tomorrow though, since I'm going to sleep now. )