First OS4 port by Thematic

MP3Gain analyzes and losslessly adjusts mp3 files to a specified target
volume. It does not simply do peak amplitude normalization. Instead, it
performs statistical  analysis to determine  how loud the file actually
sounds to the human ear.

mp3gain version 1.5.2
copyright(c) 2001-2009 by Glen Sawyer
uses mpglib, which can be found at http://www.mpg123.de
Usage: mp3gain [options] <infile> [<infile 2> ...]
options:
        -v - show version number
        -g <i>  - apply gain i without doing any analysis
        -l 0 <i> - apply gain i to channel 0 (left channel)
                  without doing any analysis (ONLY works for STEREO files,
                  not Joint Stereo)
        -l 1 <i> - apply gain i to channel 1 (right channel)
        -e - skip Album analysis, even if multiple files listed
        -r - apply Track gain automatically (all files set to equal loudness)
        -k - automatically lower Track/Album gain to not clip audio
        -a - apply Album gain automatically (files are all from the same
                      album: a single gain change is applied to all files, so
                      their loudness relative to each other remains unchanged,
                      but the average album loudness is normalized)
        -m <i> - modify suggested MP3 gain by integer i
        -d <n> - modify suggested dB gain by floating-point n
        -c - ignore clipping warning when applying gain
        -o - output is a database-friendly tab-delimited list
        -t - writes modified data to temp file, then deletes original
             instead of modifying bytes in original file
        -q - Quiet mode: no status messages
        -p - Preserve original file timestamp
        -x - Only find max. amplitude of file
        -f - Assume input file is an MPEG 2 Layer III file
             (i.e. don't check for mis-named Layer I or Layer II files)
        -? or -h - show this message
        -s c - only check stored tag info (no other processing)
        -s d - delete stored tag info (no other processing)
        -s s - skip (ignore) stored tag info (do not read or write tags)
        -s r - force re-calculation (do not read tag info)
        -s i - use ID3v2 tag for MP3 gain info
        -s a - use APE tag for MP3 gain info (default)
        -u - undo changes made (based on stored tag info)
        -w - "wrap" gain change if gain+change > 255 or gain+change < 0
              (use "-? wrap" switch for a complete explanation)
If you specify -r and -a, only the second one will work
If you do not specify -c, the program will stop and ask before
     applying gain change to a file that might clip

Source included.