Custom SOPC Builder Components in Quartus II
Check Avalon-MM and Avalon-ST specifications here: http://www.altera.com/literature/manual/mnl_avalon_spec.pdf. You'll also find all the timing diagrams in this PDF file. Usually, the control interface is done using Avalon-MM interface, which has 32bit read and write data ports, plus 32bit address, plus control signals and IRQ if needed. Avalon-ST is used for streaming devices, so You can use it for Audio.