I'm thinking about getting this pack soon, but I'd like to know more about the included music. Can someone tell me which games' soundtracks are included, and whether they're high quality versions or not?

The soundtrack download includes 34 tracks from TIM and TIM3 in MP3 VBR format with the bitrates averaging 160-200 kbps with a total play time of 1 hour 23 minutes. The quality sounds good to me.
There is a slight problem with two of the songs in the soundtrack download http://www.gog.com/en/forum/the_incredible_machine_mega_pack/problem_with_one_tim1_song

TEMIM (Even More Incredible Machines) and TIM3 doesn't include the CD audio in game http://www.gog.com/en/forum/the_incredible_machine_mega_pack/activating_cd_audio
This post says Contraptions contains in game audio which is from TIM2 (TIM 3 is a remake of TIM2) http://www.gog.com/en/forum/the_incredible_machine_mega_pack/activating_cd_audio/post5

Also note that this pack doesn't include TIM2 even though the description says it does http://www.gog.com/en/forum/the_incredible_machine_mega_pack/incorrect_game_description_there_is_no_tim2_in_this_pack

I found a fixed version of TIM3 song Unplugged. Sadly, no luck about TIM Reggae. I could upload it here, but GOG allows only images as attachment...

Where am I supposed to put the mp3's to make them work with the games? I don't see any instructions...?

...or, is this just supposed to be a soundtrack to listen to?

Thanks!

Just to listen to.