I am playing around with soundflower and aka.speech to modify the system sounds before outputting them. It is working fine except for 90% of the time I am getting a slight "echo" where a sound in the final word is respoken after the word itself.
The soundflower setup:
Soundflower 2ch is set as the output in the preference pane. Soundflower is set as the source in the DSP box and the output is internal speakers. The driver is core audio built-in output.