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There's a long backstory to this. The first Heretic mix was done in September 2001, Mus_E1M1.mus. Previous to that there was a Soundfont General MIDI bank, Silverspring. It was made specifically for playing Heretic/Hexen MIDI files (hence the name, Silverspring is the Heretic main character's hometown).
The aim of it all was a realistic soundtrack to replace the MIDI files which didn't play all that well on common soundcard MIDI synths.
With Heretic, Doom, and Hexen source code released, Doomsday became an option for playing, and it supported external files for the soundtrack.
Most of the soundtrack was finished many years ago, but some pieces were used for testing (and tuning) the new Solar Battery drumkit.
Sometime in 2012 it became obvious there's no decent (high-res) general MIDI/GS drumkit out there, hence Solar Battery appeared. Kevin Schilder uses almost all the GM percussion (except really exotic stuff like cuica), and as it turned out there aren't many high-res drumkits that actually have complete GM tom and bass kick mapping, yet alone the full percussion set. Hence it all had to be sampled. At home or at a studio whenever possible.
Some expensive higher-res samplers, although claimed to having been recorded with expensive Neumann microphones and so on, in reality get a very cold/thin sound with instruments like the cowbell. That was quite frustrating, and yet another push for the Solar Battery drumkit.
The new, 96/24 mixes, the ones with the new drumkit in it, have been made with as many virtual synths or 96/24 samples as possible.
The new files are in 96/24 FLAC (free lossless audio codec) format. Older mixes can be anything, typically 44/16, though there're 48/24, 48/16 and 44/24 files sometimes.
A good soundcard (or sound interface) and speakers/headphones are recommended. In the very least, it ought to be able to play 96 KHz/24-bit (some consumer soundcards will report compatibility but fail to play 96 KHz or 24-bit, falling back to something like 48/16).
FLAC Players
Windows: Foobar2000 with SRC, XMPlay (local), Winamp with Ota-chan's ASIO plugin (recommended), VLC.
MacOS: Cog, VLC. Avoid Themtunes - it quantises everything to 16-bit at the output stage, so any FLAC or Ogg plugin output, while working honestly at 24-bit, will still be corrupted.
Linux: plenty of FLAC players out there, though you might have to to go mucking in OSS setup to disable auto-resample to 48/16.
So, why 96 KHZ/24-bit?
Long story short, music plays like music rather than sound in 96/24. 44/16 is dull, flat, hollow, etc. In short, CDs are lacking the realism and artistic impact of higher-res digital formats. Or open reel tape, or vinyl, whatnot. CD Audio is just an obsolete format, incapable of transmitting high frequencies accurately (where the expression is), or paint delicate low-volume passages with enough detail (crucial for classical music - listen to a high-dynamic-range classical album on vinyl or in high-res digital, then on a CD). Loudness wars are in many ways a consequence of Redbook audio lacking enough warmth unless waveforms are squeezed into the top 12 dB of dynamic range (it's 16-bit only in the 0 to -6 dB range, -6 dB to -12 dB already is 15-bit, and so on, 1 bit per approximately 6 dB of audio resolution, the muter, the blurrier).
A lot can be said on why and how CD audio's supposed quality is a swindle. Really it's best for the format to die off. And you can listen to the the high-res tracks, they speak by themselves.
The FLAC files can work with ZDoom, just pack everything into a file called Music.pk3, which is really a renamed Music.zip with \Music\*.flac inside. ZDoom supports FLAC playback and 24-bit output.