Try using the "ym" sound driver instead:

ym0 at isapnp0 port 0x220/16,0x530/8,0x388/4,0x330/2,0x370/2 irq 5 drq 0,1
ym0: OPL3-SAX Sound Board : OPL3-SA3
audio0 at ym0
joy0 at isapnp0 port 0x201/1
joy0: OPL3-SAX Sound Board 
joy0: joystick not connected
isapnp0: <OPL3-SAX Sound Board, YMH002F, , > port 0x100/8 not configured
wdc2 at isapnp0 port 0x1e8/8,0x3ee/1 irq 11
wdc2: OPL3-SAX Sound Board 

Unfortunately, the driver currently doesn't seem to attach the OPL interface
(kind of ironic, considering the audio chip), but the audio portion works
fine.