Using ffmpeg to reduce mp3 file size

ffmpeg -i <output-filename.mp3> -b:a 128k <input-filename.mp3>

ffmpeg
-i file   # input file path
-b:a 128k # b for bitrate, a for audio stream. 128k defines the target speed of 128 kilobits per second, a standard balance between quality and size
BitrateQuality LevelFile Size
320kHigh (Near Lossless)Largest
192kStandard HighModerate
128kGood (Standard)Small
96kLow (Talk/Podcasts)Smallest