When i start the original game, the image looks amazing, the music is great on the menus, but when i start playing, the music becomes a disaster! I don't have any problems with the Nitro Pack! It sounds great! But in the original game I have this problem when I'm playing, it sounds very slooowwww and the voices the same!

How can I solve this? :)
thanks in advance

i have this same issue, help would be awesome

I don't know how to resolve this issue, but if you haven't done so already, you should submit a support ticket to GOG.

You might also want to check out some of the other threads about getting the game running properly, for instance this one:

http://www.gog.com/en/forum/interstate_76_arsenal/updated_new_i76_arsenal_launcher_with_automatic_workarounds_2_versions

These mostly relate to graphics and gameplay speed problems, but there's a chance it will help with your sound issue as well.