Final answer:
The term used for the organization of beats into regular groups is rhythm, differentiating strong and weak beats. Timbre describes the character of sound that distinguishes different instruments playing the same note. Loudness is measured in decibels and is distinct from timbre and pitch.
Step-by-step explanation:
The term used to refer to the organization of beats into regular groups of strong and weak ones is rhythm. Rhythm is different from a beat, as the beat is the basic unit of time in music, while rhythm is the pattern made by the arrangement of stressed and unstressed beats. Timbre is an attribute of auditory sensation which allows the ear to distinguish sounds which have the same pitch and loudness. Timbre is what makes the same musical note played on different instruments sound unique, often described with terms like warm, bright, or rich.
Regarding the question on the loudness of sound, the correct term for this is decibel or intensity level. Loudness is primarily determined by the amplitude of the sound wave and is measured in decibels. To exemplify, while both a flute and a trumpet can play the same note at the same volume or intensity level, they will still sound different due to the differences in timbre.