Different albums and types of music are recorded at different levels. When you play a collection of MP3 files encoded from different albums, comparative volume levels can vary pretty widely from track to track.
To compensate for this, SoundPlay respects the Audio:VolumeAdjustment attribute, which can be set via the AudioInfo plugin.
Keep the AudioInfo plugin window open, and if a track sounds too loud or too quiet, adjust its volume (anywhere from -4 to +4) with the picklist. The SoundPlay volume will change immediately, and the normalization adjustment will be saved into that file’s attributes. Since the data is attached to the file itself, it will be in effect no matter what playlist the file appears in.