Posting this here as well since you've posted your question on other boards. <grin>
Here's how to access the menu and increase the bluetooth microphone volume on your '08 Highlander.
Note that I've been successful doing this with my '08 HL Sport with nav and without Smart Key; also note that there are plenty of other buttons in these setup screens, some of which can really screw things up, so resist the temptation to "play around."
1) Foot off brake, all doors closed, cell phone with you in the car so it can sync to bluetooth
2) Set headlight switch to off/DRL
3) Turn ignition to "on" position - do not start the car. Wait for the nav disclaimer screen to appear.
4) Press and hold the "info" button, which is one of the nav buttons next to "disp."
5) While holding the "info" button, click the headlight switch back and forth from AUTO to OFF/DRL three times, about one movement per second. The sequence starts with the switch in the OFF position (step 2), and goes AUTO, OFF, AUTO, OFF, AUTO, OFF.
6) If you've been a very good boy or girl, your nav screen will change to a "System Check Mode" screen. Press MENU in the upper right hand corner.
7) From the Diagnosis Menu screen, press "Bluetooth TEL Check."
8) From the Bluetooth Handsfree Check Menu, press "Handsfree Voice Quality Set."
9) Now press the up arrows at "Send Voice Level" to adjust up from zero. I'm going to try a setting of 2 - I've heard that any higher settings than this and your voice will be louder but distorted.
10) Press "Set."
11) Back out of the menus as far as you can, then turn off the car.
12) Try out your new volume setting for the microphone!