In two experiments, we examined whether or not pairs of auditory dimensions--timbre-loudness (Experiment 1) and timbre-pitch (Experiment 2)--interact in speeded classification. Subjects classified values from one dimension while the other dimension was (1) held constant (baseline), (2) varied orthogonally (filtering), or (3) correlated linearly. The subjects showed substantial Garner interference when classifying all dimensions--that is, poor performance at filtering relative to baseline. Timbre and loudness displayed redundancy gain (i.e., performance faster than baseline) when correlated positively, but redundancy loss (i.e., interference) when correlated negatively. Timbre and pitch displayed redundancy gain however dimensions were correlated. Both pairs of dimensions showed substantial effects of congruity: Attributes from one dimension were classified faster when paired with "congruent" attributes from the other dimension. The results are interpreted in terms of an interactive multichannel model of auditory processing.