Dielectric relaxation spectroscopy has been used to investigate the normal-mode relaxation in disordered diblock copolymer melts far from the order-to-disorder transition (ODT). The dielectric spectra are analyzed in order to quantitatively obtain the distribution of relaxation times in the disordered diblocks. The width of the relaxation function shows significant broadening relative to the respective homopolymer distributions when the temperature is decreased and/or the molecular weight is increased. The broadening is attributed to composition fluctuation effects on the normal-mode relaxation. These effects are theoretically accounted for by considering both the short-range fluctuations due to chain connectivity and the long-range concentration fluctuations in diblock copolymers due to the proximity to the ODT and their coupling to the individual block segmental dynamics. Theory not only captures the relative features of the distributions but also quantitatively compares very well with the experimental dielectric spectra.