A method of mixing digital audio uses a plurality of mixing buses. Each mixing bus receives at least one input digital audio signal via a respective input. A sample value (801-805) of each input digital audio signal is stored in shared last level cache in the CPU. Then, for each unique input to the mixing buses in turn, the sample values (801-805) of the input digital audio signals are written (806) to a contiguous portion (808) of the shared last level cache. Then, for each input of each of the mixing buses in turn, the sample value for the corresponding input digital audio signal is added to an output value for the bus (809). When complete, the respective output values (810) for each of the mixing buses is then written to shared last level cache.