I found this PCI sound card with S3 SonicVibes chipset at the thrift shop a while ago. I couldn't find anything about it on these forums, so it must be either extremely obscure or extremely crappy. I decided to give it a try and write this little review.
Its DOS features are, according to the specs:
- SoundBlaster Pro compatible (probably means SBPro2)
- "S3FM" OPL3 clone
- General MIDI synth with chorus and reverb effects
- MPU-401 compatible MIDI/game port
- SRS 3D stereo spatializer

The card in question is a "DataExpert MED6617", made in Taiwan.

Sound quality:
Noise level is pretty good on this card! But the line-out signal is very weak too. All in all I think the signal to noise rato is not too bad. I've certainly heard worse! The amplified output is closer to normal line-level (2 Vrms), but it's slightly more noisy too so I wouldn't recommend using it as line-out jack.
A digital low-pass filter is used on the output, also known as a brick-wall filter. This means that, for example at 44kHz sampling rate, all audio above 22kHz is removed. Normally with high sampling rates this would be a good thing, but at the low rates used in DOS games, makes it sound very "dull" or "muffled". It's a matter of personal preference maybe, but I certainly do not like it.

From the spectrum plot below, it's pretty obvious why this is called a "brick-wall" filter:

Soundblaster compatibility:
At first, none of the games I tried were able to properly use its SBPro emulation. The few that did manage to detect it, AND didn't crash outright, would only produce loud noise... then still crash anyway a few seconds into the game. Here is an example:
https://app.box.com/s/p13ksurqfnar33lzhrgn (Cover your ears and have fun trying to guess which game it is! I think it just played a random part of memory there, which by chance happened to include some sound samples)

Turns out I had to load another driver for that, after doing so the SBPro emulation appears to work correctly. I can't say much about compatibility so far, but most games seem to work okay with it.

This is what it sounds like in a game with only PCM audio:
Epic Pinball: Enigma gameplay

OPL3 compatibile synthesizer:
The S3FM synth sounds remarkably different from anything else I've heard so far. Compared to the OPL3, it is FAR from accurate. But that's still no reason for me to discard it immediately. If it sounds any good, this might provide a way to experience your games in an entirely differnent way, right? (plus you'd have a whole new range of sounds in adtrack2 to play with)
Problem here is the low sampling rate it runs at, about 22kHz it seems. This, combined with the brick-wall filtering going on, makes it sound really crappy. If you try to ignore that, the synth itself can actually sound pretty good in some cases. In most cases not, though. The noise generator is way too loud, for example, and some instruments are so different they will sound completely out of place.

Here's an example where it sounds very different... but not necessarily worse, in my opinion:
Tyrian: Somebody pick up the Gryphone, Space Journey 1

In this recording the difference is even more pronounced, but in this case I can't say I like it.
Jill of the Jungle: Spiders

And an example of the noise generator going haywire:
Tyrian: Space Journey 2 excerpt

General MIDI synthesizer:
I think most of you would find this the most interesting aspect of this card. This GM synth supports 32-note polyphony and simple reverb/chorus effects. The drivers include three sample banks by The Fat Man, in 1, 2 and 4MB versions.
Guessing from the frequency spectrum, the 2 and 4MB banks use a sampling rate somewhere around 32kHz.

Here are some examples using the 2MB sample bank, with reverb and chorus enabled:
Doom: Intro, E1M1, Intermission (2MB)
Duke3D: Grabbag (2MB)

With the SRS spatializer this sounds... different. I can't really say it's better or worse, please decide for yourself. Main drawback is that it's VERY noisy.
Doom: Intro, E1M1, Intermission excerpt (2MB, SRS)

I think it sounds pretty decent overall, but on Intermission from Doom the reverb effect becomes a bit annoying, especially with SRS enabled. (The driver readme notes: "[Isseus] 1. When reverb is truned on, there is some noise." No shit!)

Note also that the SRS effect can be adjusted a bit. I left these settings on default (100%)

Some recordings of the 4MB patch set. I had expected a large jump in sound quality, but this bank merely sounds a bit different, it seems. I think I prefer the 2MB set in Doom here, because it sounds a bit more "crisp".
Doom: Intro, E1M1, Intermission excerpt (4MB)
Duke3D: Grabbag (4MB)
Duke3D: Stalker (4MB)

I'm hearing some weird popping sounds in Stalker though, and very bad distortion at the end of Grabbag. Maybe this 4MB bank is corrupted somehow?

Here's a track where I definitely prefer the 2MB patches. On 4MB the main melody is barely audible!
Blood: CBLOOD7 (4MB)
Blood: CBLOOD7 (2MB)

Now since the sample banks are loaded from software, and the driver appears to support up to 8MB banks, it might be possible to load your own samples in there. This would provide a way to use custom sound fonts in DOS games!
I don't know about the file format, however. S3 calls this system "InfiniPatch" but I've never heard of that, and google is clueless too.

Drivers:
This is where things usually go downhill with PCI cards and this card is no exception. The Win9x/DOS drivers consist of several programs and files, for DOS you'll only need:
- S3LEGACY.COM, the main driver. Takes 3120 bytes of memory, but can be loaded high. Works in real mode. One thing I don't like about it is that it modifies the SET BLASTER line in your AUTOEXEC.BAT every time you run it, and there's no way to disable this behaviour!
- S3DOSVM.EXE, if your mainboard chipset does not support DDMA you'll need to load this for SBPro2 support. The S3LEGACY driver will inform you if you need it. Takes another 1696 bytes, but will load into UMB automatically. Main downside is that it requires MS EMM386 to be loaded, JemmEx didn't work. This will eat another 4336 bytes that can't be loaded in UMB, plus you're kicked out of real mode.
- S3GM1.BIN, 1MB sample bank
- S3GM2.BIN, 2MB sample bank
- S3GM4.BIN, 4MB sample bank. The GM sample banks must reside in C:\WINDOWS\SYSTEM. If you don't have Windows installed, like me, you'll need to make a dummy directory with these files in it.
- S3MIXER.EXE, the mixer program. Looks okay at first, but after only a few minutes of use you'll notice that it's horribly broken! SETUPSA is a user-friendly program, in comparison. Most settings do not get saved correctly while some do, and many settings are always reset to default after a reboot. The mixer supports changing settings from the command line, so a possible workaround would be to write a BAT file with your preferred settings.

This review is not finished, I will update this post as I try more games and make new recordings.