Is there a utility that can enable MT-32 quality sound on this?
In a nutshell.. nope.
Some people are working on a DOSBOX-expansion to try and 'emulate' the MT-32, but so far there's a LOT to be done before that's worthwhile.
I have a soundfont for the basic MT-32 map, but that'll only give you proper sound for KQ4, LSL2 and Kyrandia 1 (maybe one or two others).
My advice is to upgrade. Obviously you want better than Adlib sound. Buy a LAPC-1 if you're money conscious..
Ebay is your friend.
To answer your question fully; the only thing with real MT-32 quality sound is a MT-32 itself (or a LAPC-1, which is the MT-32 on a sound card). Nothing else will do, I'm afraid.

A side question; which games are you trying to play?
And does your Yamaha card run General MIDI in later Sierra games?
- Alistair