To my delight, after building a kernel with sound support enabled
for the SS2 and rebuilding all devices, I am able to cat
files to /dev/audio and I can hear the sound!

Now I'm going to see if I can use software like mpg123 to decode
files.... I'm not sure what the limitations are, but at 25 Mhz I won't
be too surprised if compressed audio is a challenge.

For those who might be interested, here is the dmesg output:

NetBSD 1.5 (CLONIX) #1: Sun Jan 13 11:08:39 PST 2002
    gross@sparc2.xinetd.com:/usr/src/sys/arch/sparc/compile/CLONIX
total memory = 127 MB
avail memory = 122 MB
using 128 buffers containing 512 KB of memory
bootpath: /sbus0/esp0/sd@5,0
mainbus0 (root): SUNW,Sun 4/75
cpu0 at mainbus0: cache chip bug; trap page uncached: W8601/8701 or MB86903 @ 40 MHz, on-chip FPU
cpu0: 64K byte write-through, 32 bytes/line, hw flush: cache enabled
memreg0 at mainbus0 ioaddr 0xf4000000
clock0 at mainbus0 ioaddr 0xf2000000: mk48t02 (eeprom)
timer0 at mainbus0 ioaddr 0xf3000000 ipl 10 delay constant 17
auxreg0 at mainbus0 ioaddr 0xf7400003
zs0 at mainbus0 ioaddr 0xf1000000 ipl 12 softpri 6
zstty0 at zs0 channel 0
zstty1 at zs0 channel 1
zs1 at mainbus0 ioaddr 0xf0000000 ipl 12 softpri 6
kbd0 at zs1 channel 0 (console input)
ms0 at zs1 channel 1
audioamd0 at mainbus0 ioaddr 0xf7201000 ipl 13 softpri 4
audio0 at audioamd0: full duplex
sbus0 at mainbus0 ioaddr 0xf8000000: clock = 20 MHz
dma0 at sbus0 slot 0 offset 0x400000: rev 1+
esp0 at sbus0 slot 0 offset 0x800000 level 3: ESP100A, 20MHz, SCSI ID 7
scsibus0 at esp0: 8 targets, 8 luns per target
le0 at sbus0 slot 0 offset 0xc00000 level 5: address 08:00:20:0f:ef:a8
le0: 8 receive buffers, 2 transmit buffers
cgsix0 at sbus0 slot 2 offset 0x0 level 7: SUNW,501-1481, 1152 x 900, rev 1 (console)
cgsix0: attached to /dev/fb
fdc0 at mainbus0 ioaddr 0xf7200000 ipl 11 softpri 4: chip 82072
fd0 at fdc0 drive 0: 1.44MB 80 cyl, 2 head, 18 sec
scsibus0: waiting 2 seconds for devices to settle...
probe(esp0:3:0): max sync rate 4.03MB/s
sd0 at scsibus0 target 3 lun 0: <CONNER, CP30540  SUN0535, B0CD> SCSI2 0/direct fixed
sd0: 517 MB, 2242 cyl, 6 head, 78 sec, 512 bytes/sect x 1059528 sectors
probe(esp0:5:0): max sync rate 4.03MB/s
sd1 at scsibus0 target 5 lun 0: <QUANTUM, EMPIRE_2100S, 100E> SCSI2 0/direct fixed
sd1: 2006 MB, 3053 cyl, 12 head, 112 sec, 512 bytes/sect x 4108600 sectors
root on sd1a dumps on sd1b
mountroot: trying cd9660...
mountroot: trying nfs...
mountroot: trying ffs...
root file system type: ffs
init: copying out path `/sbin/init' 11