We analyse variation in vowel production within monologues produced by speakers in a quiet, well-controlled environment. We show that the first formant of monophthongs varies significantly with variation in a speaker’s relative amplitude. This has significant methodological consequences for the study of vocalic variation in the context of research on speaker style, and language variation and change. We also find that amplitude variation is used to mark the beginning and ending of topical sections within single-speaker monologues.