This was officially unveiled in their latest progress report.
http://pcsx2.net/281-q4-2016-progress-report.html
That's a major step forward for them even if it's still early and their Audio emulation is apparently better than all the PS1 emulators they compared it to.
The git builds can be found here http://pcsx2.net/download/development/git.html please note you need a legally obtained PS2 bios in order to use PCSX2.
http://pcsx2.net/281-q4-2016-progress-report.html
Previously, PCSX2 had always failed to play PSX games due to unimplemented devices that are necessary for backward compatibility. Rama had then decided to reach out to other developers to fix the problem. He found someone on the assemblergames.com forums that just so happened to be an expert on PS2's IOP sub bus hardware: wisi! Wisi implemented the missing PGIF device, added the required hardware hooks and with that PCSX2 was finally able to boot into PSX mode!
However, most of the PSX games still suffered from bad audio and the source of the issue was the SPU2 plugin. It was assumed that in the backward compatibility mode of the SPU2, the hardware provides a mapping window into SPU2 RAM, which wasn't properly handled by the plugin. Pseudonym then fixed all the issues related to the mapping management and SPU2-X started providing much better results. At this time, he also redid the reverb algorithm (which is shared between PSX and PS2 modes) and improved it significantly towards an exact replica of the original. The following waveform comparison shows that there are still some unsolved problems with the pulse response, but the delay and feedback are spot on! It sounds much better than the older implementation and benefits PSX and PS2 games alike.
SPU2-X reverb
-PSX backward compatibility is still in its nascent state, so bug reports related to it will be dismissed, until the emulation has matured enough.
-Also quite interesting: None of the PSX emulators used for comparison produces reverb waveforms that look anything like the original hardware!
That's a major step forward for them even if it's still early and their Audio emulation is apparently better than all the PS1 emulators they compared it to.
The git builds can be found here http://pcsx2.net/download/development/git.html please note you need a legally obtained PS2 bios in order to use PCSX2.