The soundtrack on GOG.com is coded with MP2 CBR which makes in unplayable with any Apple device. The file won't even get copied on my iPhone or iPad.

The available soundtracks here should be all either in MP3 or WAV so that they would be more compatible.

I wonder if there are any more problematic music's formats on GOG.com...

Did you try using any file conversion software on the music files? They aren't copy-protected so they should convert without issue.

No. Reason: No copy-protection doesn't protect from loosing quality by converting one lossy format to another. This is particularly significant in this case because the files are just 160 kbps CBR - two times lower than standard 320 kbps.

I use this 3rd party player: http://itunes.apple.com/app/goodplayer/id416756729

It can handle many formats and mp2 is mentioned in the product description.