You can add multiple .mp3 files from a folder with open folder or drag and drop and simply click Process.
MP3 Quality Modifier allows you to change bitrate mode (constant, average, variable), change KB rate or change sample frequency from 8.000 to 48.000 Hz.
Also, you can change the sound type for final files: mono, stereo, joint stereo or dual channel.
Each song from the list can be listen if you double click on it (will be open with your default MP3 player). You can preview in main interface details about each MP3 file: title, artist, size, bitrate, sound type and frequency. In this way you can sort or remove some files from the list based on mentioned criteria.
Pros:Easy to use, supports batch processing (multiple files at once).
ConsIt does not have included an audio player to listen files from list, but if you double click or right click on an item you can open it with your default audio player (e.g. Winamp or what you have installed)
Can reduce size of MP3 files with 50% and will free up a lot of space from computer, also is good converter if you want to modify MP3 files properties.