By now many of you have seen of played Mega Man 9. The game, an incredibly hard and frustrating affair also has a surprisingly accurate sounding soundtrack. The Games composer, Ippo Yamda discussed how the game was composed in a recent article:
Strictly speaking the soundtrack of Mega Man 9 is not NES music, but to its very core it has been created in the spirit of NES music. We made use of a program that closely resembles the console’s sound source, producing very similar waveforms, and thereby were able to make music tracks that sound just like the original. In the days of Mega Man 1-6, you were challenged to manipulate the audio signals produced by the sound card within the range designated by the restrictions of the hardware. Naturally in making this game we had no hardware restrictions, but we stuck to the formula of three pulse wave channels and one noise channel. Within this framework we freely went about composing music.
Helpful, but still pretty vague. The gang over at the Famitracker forums debated it further.
Chibi-tech wrote:
I still have yet to play the game, but after listening to the OST, I can definitely confirm that the new songs by the inti creates sound team were not created solely with the 2A03 PSG. Instead, it’s likely they used VST instrument plugins and samplers run thru a regular sequencer.
The triangle channel is the first audibly obvious indicator, as it doesn’t feature the 16-step “aliasing” that gives the channel its subtle character. At first I thought they just used a slightly inaccurate NSF player such as GNSF (as it exhibits the same clean triangle sound). Then, I heard the pitch-sweeped triangle toms — in which the stepping magically appears. (thus proving those were sampled from recordings/emulations of their previous Megaman NES games)
Next, if you listen closely to the the noise channel (the noise-drums intro to Concrete Man’s stage is a good example) you’ll notice that the noise generator isn’t pseudo-randomized like it is on the real NES. Instead it sounds like they sampled a few single noise hits from the real thing (or from accurate emulation), then used those few sampled snippets throughout the whole soundtrack. Same holds true for the short-looped noise buzzes, where you’ll hear the same exact buzz seed even when it switches to and from the regular noise mode (normally switching between short & long looped modes would capture a distinctively different short loop each time)
However all secrets reveal themselves when you examine the tunes in a wave editor. 
For instance, MM9’s pulsewaves (noticeable when a single pulse channel plays alone with a sharp ADSR attack) have an un-PSG-like bipolar waveform. The real NES’ pulsewaves on the other hand are unipolar, which a subtle low-frequency impulse pop can be heard on more drastic attacks (though eventually DC correction normalizes the overall audio.)
Well, nevermind that some of the ADSR ramping on MM9’s instruments violate the 4-bit volume scaling and vblank-timed segmenting. The triangle channel has short yet ramped releases, too! 
On the robots’ boss track, I also noticed that the lead melody’s instrument kicks in a low-pass filter when it sustains its lowest note for a few measures before pitch-sweeping up two octaves. That sort of filtering is characteristic of samplers that employ interpolation whenever the samplerate goes below a certain threshold. Sounds like pulling the pitchbend down kicked in that sampler’s capability!
The new SFX track is rather interesting in its own way. It seems like they did use the 2A03’s PSG architecture to program the sounds. However, they also utilized some post-production editing — notably the pseudo-echo effects on some of the samples. Two of the effects have the triangle channel attenuate in volume for their echo trails — something not possible on the real thing… or at least not without making register $4011 pop the shit out of the volume.
That’s pretty neato that they went all the way to include SFX interrupts on the channels though. It makes me wonder if the soundtrack is at least synthesized via the host console’s soundchip, or if they used four separate channels of streamed audio and utilized some sort of channel muting routine that cuts the appropriate channels depending on flags set on each & every different SFX.
Of course, the average gamer wouldn’t give a rats ass about these technicalities, and I bet the the NES chiptune enthusiast or Megaman fanatic would be just as forgiving. Inti Creates sound team did a good job making the whole NES PSG simulation transparent enough for even the most discerning NES chiptune listener to enjoy. I still think this soundtrack is (aside from a few songs) an amazing palette of melodic genius. It’s at least a more faithful rendition of NES music than… saaaaay… this load of ear-infecting-MIDI-file-in-fruityloops-posing-as-NES-music crapola. And it’s definitely more faithful than having globs of reverb inexplicably drenched all over the soundtrack.
Rushhjet1 appears to have been working on his own alternate soundtrack. It appears stalled, but you can grab some NSF’s.
This project is basically a “what if i wrote the music for Mega Man 9″ idea.
These NSFs are all I have left of the project right now. Ones that are slightly older copies of songs that got lost will have an asterisk next to them.
Title
Boss Selected*
Magma Man*
Galaxy Man
Tornado Man
Splash Woman
Concrete Man*
Hornet Man
Boss
Wily Castle 1 (sounds familiar..with some new touches)
Wily Castle 2
Wily Castle 3 (may sound familiar to like 2 people)
Plug Man (very unfinished, includes some melody from the actual plug man song)
enjoy… i still plan on doing final boss, possibly finishing plug/jewel, and definitely finishing small themes (like stage select and wily castle intro).