As the older thread mentioned, the soundtracks for TIM and TIM3 are included as a bonus download on GOG.
In regards to getting this music to play in-game, this site has an installer for TIM that will rip the audio from the original CD and play it as music files via DOSBox:
http://sierrahelp.com/Patches-Updates/NewSierraInstallers.html#TIM http://www.sierrahelp.com/forums/viewforum.php?f=68&sid=82c3a1908e90d8b80a876290f9492ad0 Researching this, it looks like it may use the CueSheet function in DOSBox to point to the compressed audio files:
http://www.dosbox.com/wiki/Cuesheet Thus, there may be a way to create a cuesheet for the GOG version in order to play the bonus soundtrack files in game.
TIM3 is trickier, since it's a Windows game and won't run in DOSBox. The site with custom installers suggests sticking the original CD in the drive to play the audio in TIM3. It may be possible to burn the soundtrack files to a CD and get the game to play them back, or to use a virtual CD program to mount and play back a virtual audio CD:
http://www.phantombility.com/en/support/tutorials/audio_cd/wmp/