I have the same problem and all the individual mixers (including GMPC) are maxed out.
I don't know how else to say this, but TTS speech volume is low for Garmin on windows 7. Non TTS is louder, but its not going to bleed your ear drums.
Depending on your hardware set-up and the front-end you are using, you can set your amplifier at a louder setting and turn down the volume on your music and videos using your front end. I can only speak for OpenMobile, but this will give you an idea as far as how OM will handle GMPC speech.
GMPC Speech Detection
This will allow us to attenuate or mute any other audio, to allow GMPC to be heard clearly. No need for the user to have to turn down the volume. The front-end should be smart enough to handle that.
GMPC Voice Control
The plugin is NOT released yet, but in it's full form, it is NOT embedded (no resizing issues), but instead natively integrated into OM, giving complete access to the majority of the map, data, and events generated by GMPC. And as another spoiler... there is no need for OSK button in OM. We can detect when you select a text field within GMPC, and bring up an OSK, that is, if you prefer not to use our gesture control: