I have an ASUS 7300 laptop with an opl3-sax card by Yamaha.
I am pretty sure that sound is set up correctly.
Here is the relevant part of my /etc/conf.modules:
----------------------------------------------------------------------
alias sound opl3sa2
pre-install sound /sbin/insmod sound dmabuf=1
alias midi opl3
options opl3 io=0x388
options opl3sa2 mss_io=0x530 irq=5 dma=0 dma2=7 mpu_io=0x330 io=0x370
----------------------------------------------------------------------

and here is the content of /proc/sound:
----------------------------------------------------------------------
OSS/Free:3.8s2++-971130
Load type: Driver loaded as a module
Kernel: Linux soliton 2.2.13 #1 Tue Oct 26 14:33:48 MEST 1999 i686
Config options: 0

Installed drivers:

Card config:

Audio devices:
0: MS Sound System (CS4231) (DUPLEX)

Synth devices:
0: Yamaha OPL3

Midi devices:
0: MPU-401 0.0  Midi interface #1

Timers:
0: System clock
1: MS Sound System (CS4231)

Mixers:
0: OPL3-SAx and AD1848 (through MSS)
1: MS Sound System
(CS4231)
----------------------------------------------------------------------

Playing sound (au,mp3,midi0) works fine. However, recording sound does
not work properly. I tried to use rec (from sox) and krecord (for kde).
I can record sound, but the signal is totally disorted and there is
so much noise that the result is useless. I tried mono/stereo, different
sampling rates, but to no avail.

Recording sound under windows works, so it is not a hardware problem.
Recording sound with rec/krecord on a diferent laptop with an ess card
also works fine.