I use iDealshare VideoGo to compress MP3 files to smaller file size.
Method 1: Change audio bitrate
Click "Setting..." button to open "Advanced Settings" window;
In the "Advanced Settings" window -> "Advanced" tab -> "Audio Bitrate" box;
Double click "192K" box on the right side of "Bitrate" to select smaller bitrate that you want from the drop-down list. Or enter your desired bitrate;
Click "Apply to All" and OK to finish the setting.
Tips:
This method also suits to compress audio files in both second and third category;
This method applies to compress audio with or without changing the original audio format;
Method 2: Adjust audio sample rate
Click "Setting..." button to open "Advanced Settings" window;
In the "Advanced Settings" window -> "Advanced" tab > "Audio Sample Rate" box;
Double click "48000" box on the right side of "Sample Rate" to select smaller sample rate that you want from the drop-down list. Or enter your desired sample rate;
Click "Apply to All" and OK to finish the setting.