The PCM software mixer and PCM input splitter threads run at a priority of either +1 from the highest audio client application priority or data_thread_prio, whichever is higher. To make sure that the threads closest to the hardware have the highest priority, the client application priorities should never be more than -2 from intr_thread_priority.
For example, if the priority of the client thread is 48, the PCM software mixer thread is 49 (48+1), which is one less than the default intr_thread_priority value of 50.
For more information on data_thread_prio and intr_thread_prio, see the entry for io_audio in the Utilities Reference.