Why this game does not use chip music instead redbook audio ?
Because Henshin Engine is awesome.
The only time I was OK with an *exclusively* chip (PSG) soundtrack on a CD-ROM was when (1) all the data was used for Red Book audio accompanying cinemas and/or (2) all the data was used for compressed video...
Otherwise, I was always disappointed when a developer didn't take advantage of the opportunity for Red Book soundtrack.
Now, many games used a variety of methods to overcome data constraints...mixing some PSG tunes (for towns, menus) and saving Red Book for "more important" stages...Ys Book I & II immediately comes to mind—an excellent blending of chiptunes (towns, passwords) and Red Book (for bumping death action). Sadly, some ADPCM voice samples in Ys I & II are much lower quality than Red Book audio accompanying the cinemas...a testament to how much data is squeezed on to the disc. But would I sacrifice any of the Red Book songs from Ys I & II?
HELL NO.
In short, the Henshin Engine soundtrack blends the best of both worlds—PSG "inspired" songs that have the chiptune aesthetic at their core, but with composer free to be more creative than a purely PSG soundtrack.
I hear you, though... It would be amazing to have a complete Red Book *and* PSG soundtrack that could be switched back and forth "on the fly" (see Vasteel)... But that would be placing a *ridiculous* additional burden on an already-burdened development team.
ALSO: The reason why Vasteel offered two soundtrack options was *fundamentally* PRACTICAL: PSG tunes sped up gameplay (turn-based war simulation), which was a concern. Yes, it also offered some variety (some battles were very long, it's nice to change tunes) and finally, not everyone digs Vasteel's "space jazz" Red Book soundtrack.
I apologize for my brief response.
SUMMARY: Henshin Engine soundtrack is reason *alone* to get this game.