shtewps' image has the following settings in T7G's INSTALL menu:
Music: Microsoft Windows Sound System
Speech: Sound Blaster or Compatibles
My original retail disc doesn't have those exact options (I expect WSS was a later patch), but I use General Midi and Sound Blaster, which increased the digital voice volume to match the GM output. (Stauf's voice "Welcome to my... house." is heard with GM music on the main menu ouija board when the game starts.)
I tried setting the DOSBox MIXER SB to 100:100 before launching the game but T7G sets it to 17:17 if you select Sound Blaster Pro 2 as your card - I suspect that T7G is trying to set all the mixer levels of the SB card's outputs to its own liking but either DOSBox (0.74) or Win 7 is ignoring the request to set the SB's mixer GM volume, and that's why without setting T7G to use a plain-jane Sound Blaster you will have quiet digitised voice output. Be aware that 7th Guest has pretty awful audio mastering in general so you will have some quiet inaudible sections and some blastingly loud sections regardless, but they'll be authentic.