Hi, I know a way to compress MP3 files without quality loss. There is a tool called "Bigasoft Audio Converter". I find it in Google and it is quite helpful for me.
The tool can also help:
Compress MP3, FLAC, WAV, WMA, M4A, AIFF, MP2, OGG, AC3, RA, AU and etc to smaller file size as well as keep the same audio format.
Compress WAV to MP3, AC3, M4A or AU with almost no loss of audio quality.
Compress FLAC to MP3, M4A or AC3 with high audio quality.
Compress AIFF to M4A, MP3, AC3 and more audio format.
Compress M4A to MP3, WMA, AC3, RA and etc.
Compress WMA to MP3 with smaller file size.
Compress PCM, M4B, MPC, QCP, AMR, AWB, TTA, CUE, APE and etc to MP3.