There's a problem with the sound quality where it's lower than it should be. It sounds like it came from a flash game.

For those who know a bit about sound, it sounds like 11025 Hz, where it should be increased to 22050 Hz. Unless the problem is with the configuration file and somebody can point that out and how to fix it. The archive itself shows that the sounds are indeed 22050 Hz, so it must be an audio processing problem.

It seems that sound tracks in heroes 1 weren't processed at all. I just checked with an old DOS release of HOMM and the archive stayed the same since 1995. I don't think they touched it at all.
I could provide an archive from quite recent windows version, if you think that'd be of any help. Or even CD Tracks from windows versions. But I doubt GOG guys will do anything about it, since they have handful of other games waiting to be released. It's up to us :)

Edit: Hah, got it!. It wasn't about HEROES.AGG, after all.

It seems that the link isn't working anymore.

That's because GOG changed the forum name. Fixed it.