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Jack Orlando is released in a version called Jack Orlando Director's Cut which handles the sound problem, unfortunelately GOG has released the game in the version with the sound problem which renders in useless. The speech sound is so low it's impossible to catch the storyline. A very disappoiting download.
I had the same problem. After one hour I gave up trying to fix it.
Then I read the manual: F4 brings up the options menu, with a lot of good stuff, which are, by default, disabled (?!).
If I had known... :)