by Danlee Mitchell and Jack Logan, Ph.D.

The term for gradations of amplitude (louds and softs) in music is dynamics. Dynamic levels are often "interpreted" by performers who add subtle dynamic "shading" to music.
Dynamic levels are a natural indicator for emotional mood. Loud dynamics are associated with vigor, turmoil, conflict, valor, et al. Marches, fanfares, and triumphal music tend to be loud. Soft dynamics are associated with tranquillity, repose, calmness, sensuality, et al. Love songs, lullabies, wistful and melancholy songs and certain sacred music tend to be soft. The wrong dynamic level has the effect of making a piece of music ineffective and illogical as an indicator of mood.
Dynamics are a part of the articulation of "accent" in music. Dynamic accents (accents of loudness) breathe life into all musical passages. Agogic accents (accents of length) draw attention to specific tones and clarify those tones as distinct from other surrounding tones in a melody.
Dynamic levels are a factor in the repetition - variation - contrast process of music. Patterns of dynamic levels may be repeated in corresponding repeated melodic patterns. This supports the profile of the melodic pattern itself.
As with tempo, spoken and written terms relating to dynamics are expressed mostly in the Italian language. These terms include:

Dynamics contribute to the effect of motion in music. Dynamic levels that contrast between motives or phrases of music contribute to the sensation of motion and movement in music. A contrast of dynamic levels creates a state of change in musical material which automatically produces the effect of motion in sound. Goal-oriented motion through dynamics may be created by levels of dynamics that successively become louder, phrase after phrase, or by a gradual increase in loudness through a more subtle technique known as crescendo (the Italian term for "gradually louder").