This lecture presents selected problems of multichannel digital audio processing in the time and frequency domains using fixed-point digital signal processor. Contents of the lecture: digital filtering using FIR and wave digital filters, spectral analysis, conversion of sampling rate, and generation of sinusoidal components. Using new features of fixed-point digital signal processors, digital filtering tasks can be realized in the real time, without necessity of decreasing the cycle time. Described algorithms were implemented and tested using an experimental hardware (e.g. the first polish audio card DynaBit Studio, which was partly designed by the co-author of this paper) with fixed-point processor TMS320C542.