(Credit: Vladikcomper)
Source: GitHub Repo
Commit: 333519b

What is MegaPCM2?

Description from Vladikcomper's GitHub page linked above.
Mega PCM 2 is a DAC sound driver for Sega Mega-Drive / Genesis, completely rewritten from the ground up over its previous iterations. It offers the highest quality playback possible on the hardware, high sample rate (up to 32 kHz), pitch/volume effects and supports several sample formats (WAVE, raw PCM, compressed DPCM). It outputs the cleanest sounding samples on the hardware thanks to its buffering / DMA protection system, removing much of the "scratchy" playback many games are known for.

Mega PCM runs primarily on the Z80 CPU and is DAC only. To take full advantage of Sega's sound chip and also use FM and PSG channels, you need to run it alongside the "main" M68K sound driver. Generally, Mega PCM can be integrated with any M68K sound driver. Currently, only SMPS integration is officially provided.

In S1Fixed

In S1Fixed, MegaPCM2 is integrated directly with the stock Sonic 1 Sound Driver. As a result, some bugs and optimizations still need to be made to said sound driver to ensure the best performance possible. Necessary bugfixes will be linked below, as well as in the Bug Fixes section.

Instructions for how to implement MegaPCM2 into your own disassembly can be found on the GitHub page linked at the top. More information can be found at these forum threads:
Sonic Retro Thread
SSRG Thread