I remember, when I was a young lad thinking I could change the world, and I went on the journey to develop my own game engine, surprisingly one of the hardest parts was finding a good sound component/engine. - I'm thinking of the mind set I had 10\15 years ago
Funnily, that part was a bit easier earlier in the 90s, specially when making simple 2D games, as there weren't that many fancy effects.
A "sound engine(*)" was mostly mixing multiple samples (in software, unless you were lucky to have a Gravis, or later an AWE) and playing them over the soundcard.
Which by then mostly meant either a Sound Blaster-family card or something at least compatible with the SBPro, so with a relatively narrow number of hardware interfaces to support.
(And throw in support for digital samples on PC Speaker used in pulse-width mode to support lower end machines. Add some parallel-port home-made resistor ladder (COVOX Speech Thing compatibles). Or if you feel very fancy, playing digital samples over FM chips like AdLib to cover for older machines).
Once you got the mixing, you can use it both for music, to play MODs and other similar tracker music (by mixing instrument's digital samples at varying volumes and frequencies), and sound effect (mixing digital sound effects at various volumes, mostly based on distance).
As the games were much simpler back then, sound effect were really just that: play sample more or less loudly based on how far away they are, have different left-right volumes for stereo effect.
So most of the programming tricks boiled down to mixing fast enough on slower CPUs, smoothing somewhat the sound when changing frequency and mostly the driver to make sure it's compatible across the widest amount of cards.
Contrast with graphics programming which tried to get as many effects as possible out of the limited hardware capabilities of EGA, VGA and SVGA, requiring tons of low-level register tweaking (planar 256 color modes, latches, etc.) or complex programming (using VESA bank swapping). Getting more even more complex once you want to do software 3D.
That's probably why there weren't that many good libraries when you started: the past decade was spent putting efforts into graphics and thinking "yeah, simple sample mixing will do it" about the sound (Spoiler: no, not anymore. If you want more realistic 3D games, you suddenly need a lot more efforts into 3D audio, well beyond "closer is louder": you need to map environments, do Doppler, do echo and reverb, etc. cue in A3D and the like)
(*): for the amateurs, indie, and demoscene.
For big companies (think Sierra, LucasFilm, etc.) it was mostly about supporting MIDI and tweaking a few specific musical synthetisers (tweaking the hardware registers of AdLib's FM synth, or SysEx on Roland MT-32, etc.) or later supporting General MIDI and the music sounding crap - if you weren't using the same synth as the artist (usually a Roland SC-55), you were stuck with whatever General MIDI sound bank your sound card had to offer (often some very twangy instruments in a botched OPL clone chip, or an aweful rompler).