One way of doing it is to turn the Treble control to max, and the Bass control to minimum, on the Sound Card, etc, Graphic Equaliser.
...but this will also affect all the other sounds (engine, etc)...
Alternatively..... you could always pinch you nose whilst speaking?...(Clothes Peg?)....

...!
Paul...Over and out... 8-)...!
Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.