Getting solid information on sound cards under OS/2 has been a very frustrating experience for me. Tech support people are sometimes unclear, evasive, or incorrect, and sometimes totally unresponsive. Knowledgeable responses from the net have been rare. I've seen some reports of actively hostile tech support, such as ValueMedia, which reportedly told a customer "We don't support OS/2, so don't call back."
In general, it seems that sound card support under OS/2 is spotty at best. Many cards have either no drivers or flakey drivers, and many SoundBlaster-compatible cards don't work with OS/2's SoundBlaster drivers (one IBM employee says these were written by Creative Labs with code designed to render the drivers useless for unauthorized SB clones). Other SoundBlaster-compatible cards require initialization software which may or may not be available for OS/2. Fortunately, most of the manufacturers I contacted said they were working on OS/2 drivers, either as a first release or to fix buggy or limited drivers. Unfortunately, it's hard to say when any given "in progress" driver will become available because of the notorious slippage of ship dates in the industry. I'd therefore recommend going with a card for which OS/2 drivers are available now, if at all possible.
Many sound cards these days offer "Windows Sound System" compatibility, frequently implemented with an AD1848, CS4231, or CS4248 chip. The IBM Business Audio AD1848 driver (which comes with OS/2) frequently works with cards designed for this standard, though one post mentions adding some (unspecified) parameters to the driver for CS4231-based boards. Therefore, if you've got an unsupported sound card which does support this standard, and if you're having problems with the SoundBlaster driver, try this one instead. Similarly, the Compaq Business Audio driver that comes with OS/2 may work with many of these boards. In addition, Turtle Beach has drivers written by Crystal Semiconductor for a variety of CS-based boards on their ftp site. Check ftp.tbeach.com under /pub/tbeach/os2 for the file mctz_os2.zip if you have a Microsoft Windows Sound System, Boca Modem, Acer Magic S20, Gallant, Mozart (OTI601), Crystal Vivaldi, Thinkpad 750C, or Turtle Beach Tropez or Monte Carlo and can't get another driver to work. (These drivers are reputedly still a bit buggy and limited, and many of these boards have other drivers available.)
There's a web site devoted to OS/2 sound, and many OS/2 drivers can be obtained there. The URL for this site is http://wvnvaxa.wvnet.edu/~hrieke/US.html. A second OS/2 sound site is at http://www.hitsquad.com/smm. Yet another site that may be of interest in obtaining OS/2 driver support is the Master OS/2 Warp Update web page. This contains mostly official updates from IBM, as opposed to drivers from manufacturers or third parties. I'm also told that www.drivershq.com has OS/2, as well as Windows 3.1 and Windows 95, drivers for many products. The site http://www.40th.com has a few OS/2 sound-related utilities, including a number of mixers and a couple of MIDI utilities.
For the most part, OS/2 Warp 4.0 works with the same sound card drivers as OS/2 Warp 3.0. There are a few exceptions, though, and I've noted those in the sections on the specific boards or chipsets affected.
The VoiceType Dictation feature of OS/2 Warp 4.0 (aka "Merlin") should, in theory, work with any 16-bit sound card for which OS/2 MMPM/2 drivers that support the DART multimedia extensions are available. Unfortunately, not all OS/2 sound card drivers support these extensions; for instance, the drivers for OPTi-based boards do not support DART, though there may be workarounds for some boards (see the sections on OPTi and, if present, your specific board for details). (I've also seen some claims of success with VTD and OPTi-based boards without any peculiar workarounds, so there may be more to this than I've been able to gather.)
Some of the general information presented here comes from the March 28, 1995 issue of PC Magazine, which has a review of sound cards. Although I generally don't trust magazine reviews, this article does have a fair amount of useful information on different sound card types and whatnot, and is worth reading if you're not already familiar with the market. Unfortunately, it contains NO information on OS/2 support for the reviewed cards. Other information comes from newsgroup posts and e-mail replies to my newsgroup posts or other queries, and from files downloaded from ftp sites and BBSes. The October, 1995 issue of Computer Shopper magazine also has a sound card review, but it only looks at a few boards, and mentions OS/2 in a very peripheral way. Finally, the November, 1995 issue of Computer Shopper has an article on combination sound/fax/modem/voicemail boards, but this focuses almost exclusively upon the Windows voicemail software for these boards. There is an OS/2 voicemail web site available which may be of interest to some.