This question's been asked several times already, and I'll admit that there's no real reason for me semi-ignoring it up until now. So to clear up the matter once and for all, I'll paste what's been said in the comments thread on the site:
Einherjar wrote:It appears that it's a mix of track from Phantom the Animation, Miko Mai, and some other tracks from unknown albums. Wouldn't that be illegal though?
Message wrote:It's pretty obvious that I can only say yes to that, but then again so is the (ab)use of MBAC sprites as character images. It's a dodgy practice at best, but except for masterpieces of VN storytelling the atmosphere stands or falls with the bgm, and I didn't have time to find good public domain tunes. That would take ages, if it's even possible.
Add to that that the game is pretty bloody tiny all things considered, so I'd think that youtube does a lot more 'damage' than something like this. It's only an April Fools joke, after all.
All in all Fool's Errand is a rather pathetic piece of copyright infringement, and although we did our best to use as many original graphics as we could I did not think I could, for example, retell the NAC G666 ending in the prologue without showing the actual G666 ending stage. Thus Fool's Errand's binaries ended up being probably 50/50 original/warez.
(As discussion of warez is globally prohibited in these forums, I suggest we drop this topic and just put this one down to self-righteous hypocrisy on my part: I couldn't make a parody story based on NAC G666 without infringing copyrights on CGs, and I was equally unable to tell a not-entirely-sucking story without infringing copyrights on music. The end doesn't justify the means, I just chose to ignore the means.)
That all being said, if you liked the music used in the game I suggest you buy the Pigeon Blood OST. It's beautiful atmospheric music, quite unlike most of those run-off-the-mill I've Sound funky beat tunes.