MP3 Quality Modifier (free) does one thing--change the bit rate of your MP3 file--and it does it quickly, well, and en masse. It's the en masse part that makes this program very handy. Reducing the bit rate of files also reduces their size, so you can fit more files in the same amount of space. However, you can do this with many programs. What makes this program unique is that it performs operations on whole folders or groups of files and saves the changes to the original file, relieving you of the drudgery of loading and saving individual files.
There's no install routine for MP3 Quality Modifier. You download a zip file, extract the file, and that's where it will run from. Hence, you might want to right-click, drag and create a shortcut somewhere when you first extract the executable file.
MP3 Quality Modifier is relatively simple to use. Click the Add files or Add folder button to add some files, set the qualities for the output file (mono/stereo, bit rate, frequency, etc), select the files you want to process, then click Start process. I would've thought that all files would be selected automatically; however, that's a small nit.
Please note that while MP3 Quality Modifier can increase the bit rate of files, there's no point to the operation. You can't add back audio information that was lost during the initial compression by increasing a file's bit rate. You just get a larger file that sounds the same, or possibly worse.
Despite what I consider a bit of over-thinking in the interface, MP3 Quality Modifier can be very useful if you find your MP3 collection overflowing available space. Most MP3's bit rates can be reduced significantly before the audible quality truly becomes unpleasant to the ear.
MP3 Quality Modifier is free: however, the author will accept donations. If you find the program useful and employ it for any significant operations, by all means send a buck or two his way.