Thanks to the hard work of D-Lite (http://nfggames.com/forum2/index.php?action=profile;u=167), I followed his quick n easy guide (http://www.pcenginefx.com/forums/index.php?topic=702.0) to replace the single component that cripples the sound on the Turbo Express. In a nutshell, one single capacitor neutralizes the audio amp, so the speaker is basically silent unless you hold the machine against your ear.
There's nothing to it: Open your system (six screws) and solder a new 100uF capacitor to two ready-to-use points. I used a 16V capacitor 'cause it was available, but anything over 6V will work.
That's all there is to it. Instant volume fix!
The capacitor is placed where it won't interfere with anything. I routed the wires around the brown connector to hold the capacitor in place.