gnormalize decodes the MP3/MP4/MPC/OGG/APE/FLAC file to WAV,
then normalizes the WAV to a targeted volume level and re-encodes
it. Moreover, gnormalize can extract Audio CD track and output as
various popular audio formats (MP3, MP4, MPC, OGG, APE, FLAC, WAV)
with fast speed and high quality. gnormalize can also convert audio
format between MP3, MP4, MPC, OGG, APE and FLAC with high fidelity,
which meets your need to play and collect audio files. It can change
the encoding and Metadata (tag) properties of final normalized files.

WWW: http://gnormalize.sourceforge.net/