The expandability was undoubtedly one of the crucial parts for the success of the PC. While other systems were somehow limited at this point, this system could be virtually expanded by everything imaginable.
With the squeaky sound of the pc speaker, it comes to no surprise that some suppliers offered plug-in cards to expand the sound capabilities. While some did so with almost no success (for example SSI 2001 with SID 6581, which came from the C64 (!), or Covox with their Sound Master) one company was different: AdLib. Its company name "Ad Lib" was derived from the latin, and is an abbreviation for "ad libitum" - "at will".
Since other systems sounded more or less like the multiplicity of interconnected PC-Speakers, AdLib used the Yamaha YM3812, today commonly known as OPL2. This chip, originally used in low-end-keyboards, has got two different modes: 6 voices (+5 hit instruments) as well as 9 voices. Its major advancement was, that the user could influence the sound of his instrument through multiple options by himself. While this actually was intended to reproduce real instruments, they created a quite individual sound, that really could sometimes convince - depending on creativity of the user - as autonomous.
The flip side of the coin on the other hand was that it cost also some expenditure to let instruments sound really well on this card - and the mass of PC games used the abilities of this card only very superficial, hence often sound quite "cheesy".
The great achievement of the AdLib card was to create a mass marked, and hence also practically all later DOS-based games in the 90s still supported it. However unfortunately, AdLib thought much too late about the possibility playing back digital samples, and not little later, that should happen to be their fate...
Based on the OPL2 the card could, by using frequency modulation (FM), either play 9 sound channels or 6 sound channels and 5 hit instruments (bass Drum, Hihat, Tom Tom, Snare Drum, Top Cymbal) simultaneously. Task of this chip was to reproduce real instruments. However, on the basis of only 2 frequency generators this was quite impossible.
Additionaly, one can put a volume envelope curve over the created sound in order to imitate the dynamics of a real instrument (Attack/Sustain/Decay/Release).
As a side note, some later some software (specifically tools for the MOD playback) but also some games, applied quite similar tricks like with the PC Speaker, and so actually achieved a qualitative relatively appealing sample playback.
Purchasing the card
The AdLib card has become a collector's item and hence is hard to get and if, usually expensive. If you really want one, then go for it - otherwise get a compatible card with an original Yamaha chip. There are two known versions: the original one with a full 5 mm headphone jack and the 1990 version with a 3.5 mm jack. Both should sound and work equally.
This card originated from a Joint-Venture between IBM and Yamaha, the card itself was produced by Yamaha. Dave Contois should not be quite innocent for its appearance. While he worked at this time for Yamaha, his father had been an employee at IBM already for several years, and so probably the cooperation between the two companies occurred this way...
It was essentially a Yamaha FB-01 on a card (much like the Roland LAPC-I is an MT-32 module on a card) and inherited all of the FB-01's properties: built around a Yamaha YM-2164 it had 8 FM voices, stereo-pannable, and each voice was controlled by four operators instead of Adlib's two. Its main difference to the FB-01 was that it had no LC display and featured a few more SysEx commands the original FB-01 had not (so an FB-01 is not fully compatible without additional tools). It had an extensive built-in library of over 300 high-quality synthesized instruments ("classic Yamaha" as many have called it).
You could put two of them in a single machine and get 16 voices. Because of the stereo and 4-operator FM synthesis, you would be tempted to call it an Adlib on steroids, but it was more a MIDI module than a PC-based sound card. But even in that area it was advanced: One thing that seperated it from other MIDI-based instruments at the time was that you could detune each note in very fine increments, so at one point it gained a small devoted following of atonal composers.
External MIDI box
Sierra was easily the company that supported it the most. This is not surprising, because Sierra's entire existence was saved by IBM back in 1983 when they contracted Sierra to make the first "next generation" game for the upcoming "home" personal computer, the PCjr. In addition to 16-color graphics, the PCjr supported 3 voices plus a noise channel instead of the typical single-voice beeper. Sierra had always been dedicated to IBM after the success of King's Quest, so they likewise were one of the first to support the IMFC and also more than 4 colors on MCGA machines (PS/2 model 25 and 30, which could display 256 colors but were NOT VGA compatible, hence no 640 by 480 at 16 colors and could not run EGA games).
So why didn't the IBM Music Feature Card take off? It retailed for $600, which didn't help. It was poorly marketed by IBM, and it was geared toward professional musicians much more than gamers. For games was where it had poor software support (Sierra and Microprose are two of the only three companies I've seen support it). The YM-2164 also was under an exclusive deal with IBM, the card itself was complex, so it was hard to clone it (AdLib had it the other way around, which may have contributed to its success as a "standard").
And some suspect because it didn't have built-in reverb like the MT-32, many people may have thought it sounded "worse" than an MT-32 (if you apply the same reverb-like effect to some of these IMFC samples like "minuet" you can clearly hear it was a strong match for the MT-32).
(Information and samples kindly provided by Jim Leonard)
Purchasing the card
If you thought an original AdLib card was hard to get - the IBM MFC is a rarity! It's hard to find and will usually sell for very high prices. If you find one, you're in luck! But beware: It is only supported by a handful of games.