That's how I remember the music. Good ole AdLib. We eventually got better sound hardware (and also the Doom 95 version used the Windows 95 midi sounds...Wavetable Synth, I think it was called), but this is how I was introduced to Doom, Doom 2, Heretic, and Hexen music. Still holds a special place in my heart.
Yeah, Creative used Wavetable moniker to market their AWE sound cards back in days, so that label was used a lot after that, but they were all actually sample based synthesis cards.
In Wavetable synths, sound is produced by looping single cycle waveform (that then usually runs through common synth chain of filters and modulators) with ability to go through different waveforms in table (hense wavetable) while note is still playing either manually or via envelopes and LFOs. It was sort of digital answer to analog synths that based their sound on different type of oscillators (typically sine, triangle, saw, square and noise) giving user a lot of waveforms to experiment with.
In sampled based synthesis you use samples of real instruments and then run them additonally through modulation and filtering - this is what AWE32, Gravis Ultrasound and all those PC cards from that period (as well as numerous synths and sound modules) used.