I don't know exactly why,
but my mini plays audio without any problem by:
 audioplay hoge.wav

ttyp0:root@mini  13:00:26/050308(~)# dmesg | egrep 'audio|aoa'
aoa0 at obio0 offset 0x10000: irq 30,1,2
audio0 at aoa0: full duplex

ttyp0:root@mini  13:00:28/050308(~)# uname -a
NetBSD mini 2.99.15 NetBSD 2.99.15 (INSECURE-ZS-L2-1M-PB2-SHM) 
#0: Sun Feb 13 23:21:27 JST 2005  
makoto@mini:/export/20050210/checkout/src/sys/arch/macppc/compile/INSECURE-ZS-L2-1M-PB2-SHM macppc

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Makoto Fujiwara,